Game Over, Man!
Simply beat the game on any difficulty on all three campaigns: Marine, Alien and Predator.

Stay Frosty
Beat the Marine campaign on Hard difficulty. Don't worry, this isn't near as hard as...

I LOVE the Corps!
Beat the Marine campaign on Nightmare difficulty, which means there's no checkpoints, enemies take more damage and the AI is going to hunt you down like the crazed AI it is. Want a tip? Have patience and pay attention.

I Admire its Purity
Beat the Alien campaign on Hard difficulty.

One Ugly Mother
See "I LOVE the Corps!" for details. This is just for the Alien Nightmare achievement.

I Admire its Purity
Beat the Predator campaign on Hard difficulty.

Magnificent, Isn't It?
See "I LOVE the Corps!" for details. This is just for the Predator Nightmare achievement.


The following are all story related and can't be missed.
Club Hopper
Exit Strategy
You Have My Sympathies
Regicide
I Will Never Leave You...
...That's A Promise
One Big Bug
Get to The Chopper!
Come to Mama
Breaking Quarantine
Grunt Hunt
Under Pressure
Alien vs Predator
It Uses The Jungle
Fallen Comrade
Matter of Honor
Eyes of the Demon
World of Hurt
Breaking and Entering
Reclaimer
Extinction Agenda

Grim Reaper
In the Alien campaign, you need to harvest all 21 civilians hiding about. Here's a YouTube guide on where they're found:

Harsh Language
There's 67 audio diaries laying around. It makes sense that people leave these laying around for science and stuff. Luckily, you only need to look for these during the Marine campaign. If you miss one during the playthough, you can level select to go back and pick the ones you missed up.

Here's some YouTube videos to help:

Quite a Specimen
In the Predator campaign, you've got to collect 50 Royal Jelly Containers. Mmmm, royal jelly...

Here's a YouTube video to help:

Fortune and Glory
In the Alien campaign, there's 45 skulls to collect. Same info as above.

Here's some YouTube videos to help:
Per YouTuber: **There is another belt in the FIRST jungle on some rocks, thanks to a viewer for pointing this out ! I must have missed it editing, my bad! *

I Like to Keep This Handy
On the fifth mission, you run across an objective marker on some controls after going upstairs (you'll know the place). After you've activated everything, you'll run outside and into a narrow walkway. You should be able to get it here by shooting two aliens with a shotgun and killing them at once. You can reload the checkpoint if you fail.

Let's Rock!
On the last mission, there's a swarm of aliens coming for you at the beginning. Easily done here. Just make sure to use your shotgun's burst attack.

Elite Sniper
On the third mission, you'll meet a sniper on a roof. You'll also get your rifle here. Kill as many enemies as you can with headshots and when you're all out, kill yourself and do this section over again.

Gunslinger
Easily done in either campaign mode or survival mode. Just kill 30 enemies with a pistol.

Spin Doctor
On the last mission, simple use the battle disc to kill two aliens on the bridge. Easy.

Stick Around
Simply use your Combi stick until you get 20 kills.

Scatter Shot
Just jump in a survival match and try to get in a game with 3 players for an easy unlock.

Welcome to The War
Play the "GAME" in ranked. Everyone's a winner. Lose the match? Unlocked. Win? Unlocked. Just finish it.

Killer Instinct
If you suck at multiplayer games, you might need to practice. A good way to win if you suck is to hope you join other players that are worse than you. This one might be a little tricky. Much like all multiplayer games, be aware of your environments and be patient!

Serial Killer
Play Species Team Deathmatch because you need to get 10 ranked wins. And there's no "I" in team.

Very Tough Hombre
Eww, kill 10 enemies in a row without dying? Best thing to do is find a weapon you're good with and hide (or cloak). Remember you don't have to win, just kill 10 without dying so take your time.

Persecution Complex
As Predator, cloak and kill someone you hate 5 times without dying by them. If you're about to die by the targeted person, find a way to get yourself killed another way. Suicide helps.

The Six Pack
Play with six friends? Play with six friends. Might want to boost that through the community page.

Ain't Got Time to Bleed
Play in survivor, get yourself some stims, and heal up to 30 blocks of health when you take damage.

The Uninfected
Play Infestation mode and just run around like an idiot trying to survive. All you have to do is be the survivor. When everyone else is dead, survive for 30 more seconds to win.

Welcome to The Party and Real Nasty Habit
This is a grind and probably won't be completed by most humans or aliens. Predators need to apply. Per round, you'll get around 0-100 XP and to obtain Real Nasty Habit, you need 18,060 XP. Have fun grinding!