This is a small guide with some tips and tricks I've learned playing Shadow Warrior. New players will definitely find it useful, while veterans might be able to use it to remember a thing or two.

I'm updating this as I play more and remember stuff. Haven't played the expansions yet, so this guide is probably incomplete. Feel free to correct me.

I'm not sure if this guide actually needs images. If you think it does, drop a message - I'll be happy to oblige, when I get the time.

[Octoberh 14th, 2013]
>>> Quick Tips

Gameplay
[!] Updated the training dummies tip - turns out those coat racks are actually defensive training dummies, and punching them also restores your health. (Thanks, Anahata!)

Weapons
[+] Punching walls actively damages you - you can even kill yourself doing it. So, you know, don't.

--------------------

[August 11th, 2013]
>>> Quick Tips

Items
[+] Smoke Grenades give a heavy bonus to the attack range of your fists. (Thanks, Sig Floyd!)
[+] Since Bees have token health, Gas Grenades can down entire swarms of them in moments. Careful, though - the gas grenades are lethal to YOU as well: use them only if you have some buffer space. (Thanks, Woudo!)

Weapons
[+] You can pick up thrown shurikens by pressing "Use" on them. (Thanks, Mario Dan!)

Some of these are also available on the Weapon and Enemy sections.

>>>>> Gameplay

• Save often. You never know when you're going to get one-shot by a nasty trap or a Shadow Ninja.
  
• Horizontal doors can squish you.
  
• Weapons maintain firing modes between weapon switches. Don't leave the Rocket Launcher on the Nuke setting when switching!
  
• On the topic of weapon switching, the Melee slot (slot 1) will remember which weapon you used last - the Katana or the Fists - and will switch to that weapon when you go for it.
  
• As Lo Wang loves to say: "It's all in the reflexes". He means it.
  
• Check every nook and cranny. Hit all gongs. Use all wall-mounted decorative shields. Check elevator shafts. Check all vending machines. There are secrets EVERYWHERE.
  
• Pachinko machines can be life-savers. Try conseving them intact - they drop ammo, armor, medkits, and even fortune cookies.
  
• Try conserving those training dummies intact (both the straw dummies and the solid wooden ones that look like coat racks) - you can fully regenerate health by punching them repeatedly with your fists - similar to what Duke did with drinking fountains, only with more violence. You can regenerate your health all the way up to 200 if your health is already over 100!
  
• Explosive Barrels are sometimes needed to reach certain places as makeshift platforms - check if they're useful before exploding them. But don't forget to detonate them later - they can still explode you if you're careless.
  
• Signs are usually honest. If you read "BEWARE!" or "Luggage Compactor", you better be careful. Watch for random pools of blood or spikes on the ceiling - they are usually indicators for crushers.
  
• Trash Cans can yield some very interesting items, like Fortune Cookies and even Portable Medkits - check if they're useful before exploding them. These are also used as platforms for some of the secrets.

>>>>> Items

• Don't hesitate use your Portable Medkit if your health goes below 50. ESPECIALLY if you're playing on high difficulty levels, unless you are sure there are medkits around.
  
• If there's a box of Tools around, there's a broken vehicle somewhere. If there are more Tool boxes than you need, they might be useful on later levels.
  
• Smoke Grenades can be life-savers - ESPECIALLY on No Pain No Gain. They make you partially invisible and force all enemies to stop attacking you for the duration of it. No enemy can lock on or fire at you while the smoke grenade is in effect... except those ninjas ducking behind counters and crates. They always auto-aim on you.
  
• Smoke Grenades give a heavy bonus to the attack range of your fists. (Thanks, Sig Floyd!)
  
• Night Vision Goggles are an excellent counter against Shadow Ninjas. They negate their invisibility.
  
• Gas Grenades exist. Great for tight spaces and rooms full of ninjas.
  
• Since Bees have token health, Gas Grenades can down entire swarms of them in moments. Careful, though - the gas grenades are lethal to YOU as well: use them only if you have some buffer space. (Thanks, Woudo!)
  
• I still don't know what Flash Bombs are useful for, besides briefly blinding players.
  
• Caltrops can deal a LOT of damage VERY quicky. If a caltrop trap goes off and spray the area with these, make sure to stay clear of them.

>>>>> Weapons

• You can score free kills against ♥♥♥♥♥♥ ghosts very easily while they're leaving the fresh bodies of dead ♥♥♥♥♥♥♥ with the Katana - they're vulnerable during their animation.
  
• The fists, of all things, are a great counter against those freaking BEES, due to their extreme attack rate and the bees' token health. Just aim in their general direction while spamming punches.
  
• Punching walls will actively damage you - you can even kill yourself doing it. So, you know, don't.
  
• Shurikens are one of the most accurate weapons in the game - second only to the Railgun. Don't be afraid to use them on ninjas firing from alcoves. They are also quite plentiful - almost all ninjas drop them - so don't worry if you run out of them.
  
• You can pick up thrown shurikens by pressing "Use" on them. (Thanks, Mario Dan!)
  
• The Riot Gun is great against those pesky piranhas - one shot per fish is usually enough. Wait until they're close enough.
  
• The Uzis are horribly innacurate, but there's usually a ton of ammo for them all over the place. They're useful against ♥♥♥♥♥♥♥, though - they sometimes get stunlocked.
  
• The Rocket Launcher is somewhat innacurate - keep that in mind when firing from cover, or you'll end up blowing yourself.
  
• It's easy to forget that the Heat-Seeker card for the Rocket Launcher lasts only for 5 rockets. Save these for the Shadow Ninjas.
  
• Nukes are extremely situational. Use them only when cover is available, and only when you actually have enough time to reach said cover. They kill everything caught in the blast radius, so a direct hit is not necessary (as long as you don't launch it into an air vent or something), and deal heavy damage to bosses.
  
• Grenades from the Grenade Launcher have an ENORMOUS blast radius. This makes grenade traps specially deadly, since they spray these like confetti. This fact also makes Gray Ninjas particularly dangerous.
  
• You can tag enemies with Sticky Bombs. Multiple stickies. The first to go off with detonate the others as well. You can also shoot them yourself.
  
• The Railgun is slightly misaligned ingame - try aiming slightly to the right when aiming at medium to long-range targets.
  
• The Guardian Head is devastating, but chews through ammo REALLY fast.
  
• Don't be shy to use Ripper Hearts. They're great when you need that extra firepower, and are more plentyful than you may think.

Every weapon in the game is useful in one way or another.

>>>>> Katana

Shadow Warrior's iconic weapon. Deals heavy damage and has good reach.
Great Against: Brown Ninjas and crossbow ladies. You can score free kills with it on freshly-spawned ♥♥♥♥♥♥ Ghosts.

>>>>> Fists

Fast-firing melee weapon, with VERY short reach. Second only to the Uzis in fire-rate.
Great against: Bees.
Use: You can regenerate health by punching training dummies.

>>>>> Shurikens

Second-most accurate weapon in the game - throws 3 shurikens at a time, one right down your crosshair, one to the left and one to the right at an angle. Abundant ammo.
Great against: ninjas at mid-range. Also good on close quarters, where all 3 shurikens hit the same target.

>>>>> Riot Gun

Shotgun. Slow fire rate. Tight spread. Decent damage. One-shots brown ninjas. Reloads every 4 shells. Ammo pickups are scarce, but give lots of ammo.
Alt-Fire: Full auto: fires all 4 shells in rapid succession, at the cost of accuracy.
Great against: most enemies, especially ♥♥♥♥♥♥♥, Piranhas and Rippers.

>>>>> Uzis

Bullet hose. Innacurate. Bullets are not hitscan. Reloads every 50 bulles. Abundant ammo.
Alt Fire: Dual-wield. Requires another Uzi. Reloads every 100 bullets.
Great against: most enemies, especially ♥♥♥♥♥♥♥ and all Rippers.

>>>>> Rocket Launcher

Standard explosive delivery. Slightly innacurate. Packs a punch. Rockets are somewhat hard to come by.
Alt Fire 1: Heat-seeker missiles. Requires a Heat-Seeker Card.
Alt Fire 2: Nuke. Devastating, but you can only carry one.
Great against: Turrets, Bosses, Shadow Ninjas.

>>>>> Grenade Launcher

Standard explosive delivery. Grenades bounce on surfaces, explode on contact or after some time. Surprisingly large blast radius. Can start fires. Ammo pickups are scarce, but give lots of ammo.
Great against: Rooms full of enemies and enemies behind cover.

>>>>> Sticky Bombs

Thrown, proximity bombs. Arm after a few seconds. Can be manually detonated with weapons fire. Boxes of ammo are hard to come by, but you won't be using them much anyway.
Great against: enemies that run all over the place.
Use: You can tag enemies with them.

>>>>> Railgun

Sniper rifle. Dead accurate, if you compensate for the weapon's misalignment by aiming slightly to the right of your target. Ammo is very hard to come by, but you won't be using it much anyway. Keep in mind that there's a slight delay between pulling the trigger and actually firing. Be sure to take that into consideration.
Great against: multiple enemies in a line, very distant ninjas.

>>>>> Guardian Head

Flame thrower. The weapon Shadow Ninjas are infamous for.
Alt-Fire 1: creates a persistent ring of fire around you, deals heavy damage. Lasts for a few seconds.
Alt-Fire 2: Fire bombs. Devastating. Uses a LOT of ammo.
Great against: Shadow Ninjas, Bees.

>>>>> Ripper Heart

Creates a shadow copy of you, equipped with a Railgun. Vanishes after a minute or so.
Great against: Non-boss Snake Gods and Sumo Wrestlers.

A quick description and some tips on dealing with them.

>>>>> Brown Ninja

Standard mook - the foot soldier of Zilla's troops. All ninjas can duck, climb and jump around, as well as take cover. The brown ninjas can attack with both Uzis and Shurikens. They commit suicide every now and then.
Strategy: The Katana and the Riot Gun can both one-shot them.

>>>>> Red Ninja

Tougher version of the Brown ninja - they can also maneuver around, but instead of Shurikens, Red Ninjas launch slow-moving ROCKETS. It takes about 3 swingsto kill these, making it dangerous to tackle them with the sword.
Strategy: Rocket Launcher or concentrated Uzi fire.

>>>>> Orange Ninja

Same as the Red Ninja, only they fire HEAT-SEEKERS.
Strategy: Treat them like Red Ninjas, but try facing them around cover.

>>>>> Gray Ninja

Same as the Red Ninja, only they fire GRENADES with a Grenade Launcher. Considerably more dangerous, since the grenades aren't slowed down like the rockets and the grenades have a bigger blast radius than them.
Strategy: Treat them like Red Ninjas - but with a higher priority.

>>>>> Shadow Ninja

Arguably the most dangerous enemy in the game. The strongest and fastest of all ninjas - he's also PARTIALLY INVISIBLE. He'll use Flash Bombs on you. They can run around very fast, can take quite a lot of punishment and use the Guardina Head. They have two attacks, besides the flash bomb: slow-moving Fire Torpedos that fire fast-moving fire darts to the sides and the dreaded FIRE BOMBS, which they'll use to carpet-bomb the whole area in front of them. Incredibly dangerous in close quarters, these guys can one-hit kill you even if you have full health AND armor.
Strategy: Explosives. Explosives EVERYWHERE. These guys have a good deal of health so bullets will take forever. It is imperative that you kill these guys AS FAST AS POSSIBLE. The Guardian Head's second alt-fire - the Fire Bombs - are especially effective against them. The Night Visioon Goggles negate their partial invisibility. They are EXTREMELY DANGEROUS - don't be too proud to back off, specially if there are several of them in one place.

>>>>> ♥♥♥♥♥♥♥

Chinese villagers turned into suicide bombers. Stay away from them and you'll be fine. They're kind of silent, so watch your surroundings. Be careful when entering enclosed spaces and corners: Their explosive charges can deal VERY HEAVY damage.
Strategy: Use the Rio Gun on Full Auto. The Uzis can stunlock them. Rocket launchers are kind of excessive, since they already explode, but gibbing them will disrupt any chances of them ressurrecting as ♥♥♥♥♥♥ Ghosts. You can also use them as bombs by forcing them to detonate while in the middle of a group of other enemies.

>>>>> ♥♥♥♥♥♥ Ghosts

A ♥♥♥♥♥♥'s last ditch effort to kill you. They are small and can turn fully invisible for a few seconds, reducing their hitboxes as well. They launch smal red balls which deal good damage.
Strategy: Wait for them to matrialize and hit them with two shells or concentrated Uzi fire. You can also kill them with two swings of the Katana. You can kill them with almost no effort while they're spawning from dead ♥♥♥♥♥♥♥.

>>>>> Female Ninja

The only females in the game besides the hentai girls and that old lady from Hara Kiri Harbor, they'll shoot you with their crossbows at mid to long range. But their true power lies in laying sticky bombs - don't let them run around for too long or there might be a sticky trap waiting for you later.
Strategy: Use the Katana to kill them fast in close quarters. Otherwise, pull out the Riot Gun or the Uzis.

>>>>> Guardian

Tough orc-like enemies. They have two attacks: a melee swipe with their blade or dual fireballs from their eyes - similar to those statue/wall traps.
Strategy: Full auto with the Riot Gun or rockets, since they have decent HP.

>>>>> Ripper

Ape-like enemies. Good health, usually come in packs. They can run and jump at you. Slash with their claws. Fun fact: they can rip your heart out if they kill you - that's why they're called Rippers. Come in two flavors: Normal, gray Rippers or giant, Brown rippers. A word of advice: Rippers run EXTREMELY FAST when set on fire. Avoid using the Guardian Head's primary fire (heh) against them.
Strategy: Riot Gun or Uzis. Abuse explosives when dealing with the giant ones.
Tip: If you gib them, they might just drop a Ripper Heart.

>>>>> Baby Ripper

Predator-like enemies. Fragile like Brown Ninjas and fast as Rippers. They attack by launching a green ball at you, similar to ♥♥♥♥♥♥ Ghosts. They can climb on walls. Come in two flavors: normal and giant.
Strategy: One shell is enough - just pop them in the face. Use full auto with the Riot Gun or even a few explosives when dealing with the occasional giant.

>>>>> Bees/Hornets/Whathaveyou

Extremely annoying, hard to hit. Come in swarms. Have the least HP and the weakest attack in the entire game, but pack quite a punch when you combine their extreme speed, their sheer numbers and their extreme maneuverability.
Strategy: Uzis, Riot Gun and/or the Fists. They're hard to hit, since they're so small. Use the Uzis on the swarm to reduce their numbers from afar, the Riot Gun to pick them off from mid-range and the Fists if they get too cloose and start circle-strafing you. You can also pop a gas grenade if you have the space.

>>>>> Piranhas/Koi

Cannon fodder. They're just annoying. Come in packs. Slowly swim towards you and tries to bite.
Strategy: Riot Gun to the face. Let them get close to one-shot them. If there are just too many of them, consider herding them in one place and using explosives.

>>>>> Accursed Head

Floating trap. Shoot them or get close to make them detonate and activate their trap effect.
Strategy: Shoot them from afar to trigger the trap safely.

>>>>> Serpent God

Small version of the Serpent God boss - like those mini Battlelords from Duke Nukem. They got dual blades and can dish out some good damage when combined with their dual green blob attack. High HP.
Strategy: Explosives. You might want to use a Ripper Heart when dealing with this guy.

>>>>> Sumo Wrestler

Small verison of the Sumo Boss - but without the earquakes and bombs. Very high HP - they can clap their hands and set you on fire. These guys.... fart when they die, essentiually detonating a gas grenade on death.
Strategy: Shoot him with everything you've got. Explosives are best, as to spare ammunition. You might want to use a Ripper Heart when dealing with this guy.

Hope this little guide helped you. Got to stop being lazy and get some screenies and pictures.

That's pretty much it. Now go stab some ninjas in the face.