So basically every achievement (except "SNOOKER KING!" achievement) is either getting 3 stars in a level (=100%) which means to complete every objective in a mission. (Usually that's 2 side objectives and the main objective, but sometimes there is just the main objective.) Accuracy and other stuff aren't important for that.
Or by collecting a medal by shooting/punching it. (Usually there are 3 medals per level, though on some levels there aren't any.)

Better play the game on normal instead of hard mode. It will be difficult enough. Also I would advise you that you first play through the game once without the guide, so you get used to the game and the maps. Also for various other reasons, like you can't get certain medals for the first time and you can't jump at first, you gain the ability to jump after finishing the mine level for the first time.

Keep in mind you can replay every level later, via the "replay" which you find in every hub and you can change the hub via the "travel" which is also in every hub. And there you can also see your progress towards stars and medals. ("replay" for the main levels and side missions, "travel" for the hubs. And yes, there are also medals in the hub areas.)

So I will tell you how to complete all these objectives and how to find the medals. The sections of this guide are the different levels of the game. First I'll go through every main level, then both side missions, then the 4 hub areas. Though this won't be the order you first play these.

Estimated time for 100% completion: maybe 5-10 hours

• THREE STARS! - TRAINING
  You get this after "killing" all targets in the training section. It's pretty easy. You can also replay this mission later. (Same with every other mission.)

• This medal is behind a president in the whitehouse.
  Almost at the start, above the stairs, shoot the picture.
  
• This medal is up high in the whitehouse.
  In the room with the big eagle on the ground and a "7", jump on the table, then on the cupboard and from there jump to the higher ground. Go in the dark room and turn right.
  
• This medal is 'elevated' in the whitehouse.
  During the countdown, when you come back to the main room, the elevator will open with a bunch of bad guys and the medal.
  
• THREE STARS! - WHITEHOUSE
  Save 3 hostages and the vice president (have to shoot/slap them), then complete the mission.
  First hostage is right at the start.
  After collecting a "7" in a room with a big eagle on the ground, an elevator will open with the second hostage.
  Third hostage is in the left wing, on the right, behind a crate.
  In the room before collecting the "7" to start the countdown, shoot this switch.
  It will open this door.
  Go through to find the vice president.
  (Thanks to Formidolosus who helped me find this one.)

• This medal is near the start at the mines.
  Almost at the start, look at this door before collecting the second "7".
  It will open after collecting it. An enemy will come out and the medal is in there.
  
• This medal is near a red barrel in the mines.
  To the left of the elektro door, next to a red barrel, destroy the wooden planks, crawl through and jump a bit. (You gain the ability to jump after finishing the mine level for the first time.)
  
• This medal is of 'alien' origins in the mines.
  In the same room where the red barrel and the planks are to get a medal, use the red barrel or a grenade and blow up the rocks to the little room. Inside is a switch, shoot it to open the eletric door in the same room.
  Kill all enemies down there, then you will see an alien spaceship behind some bars. Crouch, to get through. Behind the ship is the medal.
  
• THREE STARS! - MINES
  Find the files, save 3 hostages, then complete the mission.
  In the same area, where the alien spaceship is, go a little further to find the files. Destroy them.
  After the room with the switch, go right, punch the barricades to find 2 hostages.
  Facing the bomb while defusing or after it, go in the first tunnel on the right to find the third hostage.
  (It will open at some point while defusing and at least one enemy will coume out.)
  

• THREE STARS! - SPEEDBOAT
  Collect the 3 air drops, destroy the antenna, then complete the mission.
  All air drops are at the start, don't drive through the first gate yet, instead drive around the island on the right. When you see that lighthouse, drive to that ship on the far right to find the first air drop.
  Continue on the right side of the island. After the sea mines you find the next air drop behind a sandbank.
  Continue a bit more to find two ships. Third air drop is between those.
  The antenna is in the same area where you find your first "7". Just drive onto it to destroy it.
  

• This medal is high up at the start of the beach.
  At the start, right before going behind the building to get the first "7", go on the red barrels and then jump on the ledge. (Don't destroy or move them to much or else you can't make the jump.)
  Now go left and follow the way, at the end jump and then crouch.
  You will be a new area now. Destroy the planks in the ground to reveal a hole. Jump down. (You need a lot of health to survive that.)
  Follow the way to find a sniper which you can use to shoot the medal you can see from there.
  
• This medal is contained somewhere at the beach.
  After the big ambush of diver enemies, go right to those 2 red containers. One of them is now open with the medal inside.
  
• This medal is near running water at the beach.
  Before you repeat the steps here, better read this medal section till the end because there is an easier way for this medal which I will tell you after the hard way.
  Hard way: In the countdown section, instead of jumping over the containers, go right and jump around the corner (tough jump).
  Go behind the running water. You have to jump over the map borders. Take care to not fall of the map.
  Easy way: Just face the running water and crouch to go behind it in a little cave. There is the medal. Also you don't have to do that during the countdown.
  
• THREE STARS! - BEACH
  Destroy the 3 gun crates, save Lotta's parents, then complete the mission.
  In the first building, which opens after collecting the first "7" is a switch behind a board. Shoot it to open a door where the "7" was. Go there and kill all enemies to save Lotta's parents.
  (Thanks to Nerdy Nom Nom who helped me find that one.)
  In the first building you enter, after collecting the second "7", destroy those planks and shoot the switch.
  It will open a nearby passage.
  There are 2 gun crates there.
  In the countdown section, go left and jump there instead of jumping over the containers. Go left again to find the last gun crate.
  

• This medal is where you call people at the docks.
  After jumping down, go left to a telephone, there is a vent on the right, crawl through.
  
• This medal is near the end of the docks.
  To the left of the big crane, hanging from the footbridge.
  
• This medal is contained at the docks.
  Where you have to move a crate for the second time, instead go through a gap between the containers.
  
• THREE STARS! - DOCKS
  Destroy the 3 drug supplies and the drugs lab, then complete the mission.
  Go down where you start, then down the ramp, in the water. Now go all the way to the right into another building. There is the first drug supply in a corner.
  In the room where you activate that first crane. After it, go through that door.
  Shoot that button.
  This has opened this nearby door.
  Follow the way to find the drugs lab. Destroy it.
  From the big crane, go right, under the ramp. Destroy the planks to reveal the second drug supply.
  In the room where you have to activate the second crane, before doing that, go up the right stairs. Jump on the crate at the end to find the third drug supply.
  

Worth noting that if you want your 100% in this level, you can't die or fail after the checkpoint after beating the boss. If you do die after that point, your progress for your finished objectives will be reset due to a bug and you have to restart the whole mission.
• This medal is in the ships engine room.
  Right after you found your first "7", backtrack to where you came from.
  Another door will be open now. It is the engine room and you will find the medal there.
  (Thanks to Nerdy Nom Nom who helped me find that one.)
  
• This medal is hidden near a sailor.
  Right after you found your first "7" and gone through the newly opened door. It is behind those crates. (Also a sailor is there.)
  
• This medal is hidden deep inside the ship bowels.
  During the ending countdown, go left here. (If you go right you find another "7".)
  
• THREE STARS! - SHIP
  You have to save the 3 sailors (hostages), disable the main engines and then complete the mission.
  First sailor is next to your first "7".
  Check first medal on how to get in the engine room. Destroy the main engines.
  (Thanks to Nerdy Nom Nom who helped me find that one.)
  Second sailor is right next to second medal.
  Third sailor is during the ending countdown. Go right here and right again. (You have to go left and you will also be attacked by one guy from the left.)
  

• This medal might get carried away at the airbase.
  Almost at the start, in the second room, inside the first transporter.
  
• This medal is where you sleep in the airbase.
  In the second bedroom, where a lot of enemies come out.
  
• This medal is at the end of the airbase.
  Right after the second cafeteria turn left.
  
• THREE STARS! - AIRBASE
  Save the 3 pows (hostages), destroy the radar station, then complete the mission.
  Continue straight from the second bedroom to find the first pow.
  After you went up the stairs again, go straight here to find the second pow.
  In the same room from there, go right to find the radar station. Destroy it.
  In the first cafeteria, go left to find the third pow.
  

This whole level is timed. You have only 2 minutes, 40 seconds. Better play this map twice, once for medals, once for stars.
• This medal is near the start of the gaslabs.
  Almost at the start, in the second room with gas canisters, the medal is hanging from the ceiling downstairs.
  
• This medal is located in the security room of the gaslabs.
  Right after using the little crate, go through the vent in that room to find the secret lab and on the wall on the right is the medal.
  
• This medal is up high in the gaslabs.
  Go left here.
  After the little hallway, turn around to find the medal, up in a corner of the room.
  
• THREE STARS! - GASLABS
  You have to destroy the 3 generators, clear out the secret lab, then complete the mission.
  First generator is right at the start, turn right.
  Second generator is after using the little crate, on the right, up in the left corner.
  Then go in the secret lab via the vent and kill all the enemies there to complete that objective.
  Go right here.
  Continue on. The third generator is in the last room next to the gas canisters, up in the left corner.
  

• This medal is in the restaurant of the train.
  Right at the start, under the bar. Crouch to better see it.
  
• This medal is at the end of the train.
  After collecting the first "7". Go in the first cabin.
  Go through the vent.
  Follow the way till the last room, the communications room.
  At the far end, behind a crate is the medal.
  
• This medal is somewhere outside the trainroof.
  During the countdown section when you are walking on top of the trainroof.
  Just wait at the start. The medal is hanging from the second metal thingy which can crush you. It will come when the timer is at about 1:37. From there wait like 4 seconds so it's not to far away and shoot it. It's quite hard to hit it. Best to use is the mp.
  
• THREE STARS! - TRAIN
  Destroy the 3 trackers and the communications room, then complete the mission.
  The first tracker is in the first wagon with seats. It's on the right, under the third bench.
  Check second medal for how to get in the communications room. Destroy those papers inside to complete that objective.
  Now go back to where you picked up the "7" and go in the third cabin. There is an enemy and the second tracker under the seat.
  Now go in the next wagon and in the third cabin. The third tracker is there on the wall.
  

• This medal is outside the train on the wilds.
  Right at the start, on your way to the first "7", jump on the left snow pile and from there jump on the next one. (Tough jump)
  Now continue your way almost till the very end to find the medal on the right at the train.
  
• This medal is on a tree at the wilds.
  From here.
  Go to the right to find the medal hanging on a tree.
  
• This medal is high up in a frozen place at the wilds.
  In the area after the checkpoint where you find your next "7". Jump on that rock.
  Turn around and jump on that icy structure. You will find the medal there on the wall.
  
• THREE STARS! - WILDS
  Destroy the 3 antennas, save agent 5, then complete the mission.
  From the checkpoint, go behind the snowman. The first antenna is there.
  From the ckeckpoint, go right and under the fence. In this area is the second antenna.
  From the ckeckpoint basically go straight all the time to find a tunnel after a truck. Behind it is agent 5 hanging on a branch. Shoot the guy to save him and to complete that objective.
  During the countdown sequence you will automatically come by the third antenna.
  

• This medal is a pillar of hope at the mountains.
  Before collecting the second "7", look up on these giant rocks on the left.
  
• This medal is at the top of the world at the mountains.
  Just when you reach the top of the mountain, look in that corner.
  
• This medal is in a high up cave at the mountains.
  This is also at the top. We want to get on that ledge.
  Get on top of the mountain and then drop down to it. (Might take a few tries.) Go in that cave to find the medal.
  
• THREE STARS! - PEAK
  Destroy the 3 gun crates, beat agent 3 to the top (be on the top of the mountain before him), then complete the mission. So you basically have to speedrun this (platforming level) while getting the gun crates. There are no enemies.
  Almost at the start, go left here to find the first gun crate.
  From here, when you see the 2 "7", go right, jump over the gap and follow the way to find the second gun crate.
  Before going in that cave, look right. The third gun crate is in a little hollow in the distance.
  The only advise I will give you for the speedrun is to use the shortcut at the second gun crate.
  

• This medal is outside your reach at the snowbase.
  Near the start, next to the first "7".
  Position yourself like this.
  Then crouch and you should be able to see the medal outside the window. Though it's hard to see with those grey rocks behind.
  
• This medal is near a helicopter at the snowbase.
  In the room where the helicopter was, look way up on the ceiling.
  
• This medal is in an icy cave at the snowbase.
  In the icy cave, go down on the right to find the security room. Behind the last rock is the medal.
  
• THREE STARS! - SNOWBASE
  Destroy the 3 computers, clean out the security room, then complete the mission.
  The first and second computer are after the checkpoint, on your way to the "7".
  Check the second medal on how to get in the security room. Kill all enemies there to complete that objective.
  Go left here and left again to find the third computer.
  

• This medal is housed well
  Almost at the start, after the first fight, on the right.
  
• This medal is a bridge to glory
  In the area before the sea mines. After the first bridge, turn around. It's on the right of the bridge.
  
• This medal is near end
  When the 2 gondolas with enemies cross your way, look left. It's pretty hard to get. Best is to instantly kill the first enemy, then ignore the second and look for the medal.
  
• THREE STARS! - GONDOLA
  Just complete the mission.

• This medal will give you a bad start
  Right at the start. On the other side of where you spawn, in the water.
  
• This medal has been shelved
  In the first big wine cellar, go straight to the end. Medal is behind 2 barrels.
  
• This medal is a pile of trouble
  In the room with the first "7". In a middle of a bunch of wine barrels.
  
• THREE STARS! - CELLARS
  Destroy the drone plans and the 3 gun crates, then complete the mission.
  First gun crate is in the first big wine cellar, in a little room on the left.
  Second gun crate is next to the first "7".
  Third gun crate is in the same room, after collecting the "7". Almost go out of the room, then go left. It is behind some barrels.
  In the water area, when you get attacked by some spiders, go left and go through the vent. Behind that are 2 enemies protecting the plans. Destroy them.
  

• This medal might be outside
  On the balcony on the left, it's quite far away.
  
• This medal is in the great hall
  Go downstairs, it's hanging from the other stairs.
  
• This medal likes books
  Downstairs, go in the "files" section.
  Here go right, then right again.
  The medal is on a book shelf.
  
• THREE STARS! - ARCHIVES
  You have to save 3 agents (hostages), clear out the security room and then complete the mission.
  The first agent is just in the room before the balcony.
  Downstairs, check for a vent. Go through to find the security room. Kill everyone in there to complete that objective.
  In the "files" section, almost at the end you will find the second agent.
  Go in the maze. Go right, then left. Third agent will be at the end of that dead end.
  

• This medal is on a table
  Behind the pool table.
  
• This medal is bogged down
  In the woman toilets.
  
• This medal is in the waterway
  At the start of the water area, to the left. You can shoot it now from here from this bad angle or get it a bit later.
  Get it later: Collect the "7" in the water area to start the ending countdown. Now go back from where you came killing all enemies. To the right a door will be open. Go there, and left.
  
• THREE STARS! - SIEGE
  Just complete the mission.

• This medal loves to shop
  Next to a shop on the right side.
  
• This medal is in the access area
  After crawling through a vent, go left.
  
• This medal is somewhere low
  Right at the end of the mission. Before entering the exit, go around the hole.
  From that angle you should see the medal down there.
  
• THREE STARS! - TUNNELS
  Just complete the mission.

• THREE STARS! - FINAL
  Just complete the mission.

• This medal likes sheds
  In the right shed.
  
• This medal is near a tee-off:
  From the clubhouse, go left. Next to some pistol ammo is the medal.
  
• This medal is up high
  Right after collecting the second medal. It is on the backside of the clubhouse.
  
• THREE STARS! - GOLF
  Just complete the mission.

• THREE STARS! - BOATRACE
  Just complete the mission.

• This medal is hidden in the office toilets.
  In the man toilets, second cabinet from the right.
  
• This medal is hidden in the office labs.
  Go in the labs, push the little crate to the other side, so you can jump on the shelf. You will see a vent. Jump to it, crawl through and get the medal.
  
• SNOOKER KING!
  Shoot all billiard balls in the holes. (The only achievement of the game which is not unlocked by getting 3 stars in a level or by collecting a medal.)
  
• This medal is hidden in the office filing room.
  Go inside the replay room and shoot the picture to reveal the medal.
  

• This medal is in an alleyway at the shantytown.
  Right at the start, turn around, jump on the crates on the right, then jump on the canvas on the left. Now jump to the left around the corner over the fence. Medal is behind the crates there.
  
• This medal is somewhere at the shantytown marketplace.
  Go left at the market. Look through the slightly open gate.
  
• This medal is 'fishy' at the shantytown.
  Very similar to first medal on that level. Right before going in the apartments, jump on the crates on the right, then jump on the canvas on the left. Now jump straight over the fence. (This jump is very tough.) Medal is behind some crates there.
  

• This medal might be near the pilots cockpit on the airplane.
  Right at the start, turn around, next to the "travel" sign. You CAN'T get that one during the first time you are in the airplane. You have to go back in the airplane later on, via "travel" to get it.
  
• This medal loves tray service at the airplane.
  Almost at the start, at the first tray service on the right. Destroy it to reveal the medal behind it.
  
• This medal is on the lower deck of the airplane.
  Soon after descending the stairs, on the right side.
  

• This medal is near the bridge at the xmas village.
  Cross the bridge, go left and make your way above the pile of snow.
  
• This medal is near the marketplace of the xmas village.
  On the marketplace, right before the church, in a shed on the left side.
  
• This medal will barrel you over at the xmas vilage.
  At the marketplace, go left. Till you find a dead-end with 3 barrels. Behind the last 2 is the medal.
  

If you find any mistakes or unclarities or have questions let me know in the comment section and I'll see if I can help.

Also it would be nice if you could rate this guide positive. It took me a lot of work.