Awesome game
Tips and hints:
These are from the forums:
# Move around and jump around a lot: Really basic, since if you move around and jump, it'll be harder for enemies to hit you.
# Use the super-jump: By first jumping normally or forward, and doing a backflip as second jump, you can jump slightly higher than with a normal double-jump, hence the name super-jump. You need to do the super-jump to access the weapon rooms in Genetics Lab 2 and 4. -Wexor
# Stay away from enemies with strong weapons: If you're not close to them, the chance of being hit by them will be less, and thus, there's a better chance of surviving. -IcoShee
# Spawn count: The doors opened based on the spawn total count, not the kill count. I can even verify this. I took a picture of the game when I got to one point, and after messing around and not killing mercs, the exit opened on it's own. -DarkAce
# Double-jump to the platforms above: The enemies spawn on the three platforms, and if you just stand by one of them, and maybe crouch, you will be able to evade many enemy bullets, and can if feeling like getting the melee achievement start bashing the ones spawning at the portal you're next to with your lead pipe, and when completing the melee achievement, with your sword. You can either do this for the whole level, or just jump up there to regain health, the choice is yours. -Wexor
# Don't pick up everything you see: Really, try not to pick up guns too much, since they always are low on ammo. You can look at it this way: Just because you see an HK 416 on the floor, it doesn't mean you should switch your MP-5K with 250 bullets for it, since you'll only have around 25 - 30 bullets on the HK 416. On the other hand, if you never switch weapons, you'll never get any large number of ammunition on any weapon but the one you started to use in the beginning of the game, nor will you be able to unlock the All Weapons achievement. -The Forum Ferret II
# Don't get cornered: When avoiding enemy bullets, don't get too close to any walls, since it will be easier for them to shoot you if you're boxed in! -The Forum Ferret II
# Throw 'em to death: You can throw your weapons by pressing spacebar, and this can do a small amount of damage to the enemy (aim for the head). If you want to pick up a new weapon, but already have two, you need to throw one of your current weapons away to pick up a new one. -Tayta
# Throw away the shit you don't like: Another pretty basic one, if you want to pick up a gun but already have two guns, make sure that you throw away the gun you don't want. There's nothing worse than running around carrying junk you don't want or need. -BlueGhost
# Hard Mode: When you have unlocked Hard Mode, you have to go to Options and click "Hard Mode: On" and then click continue game to actually play in Hard Mode. And don't worry, if it gets too hard, you can switch back to normal mode any time you want by going to Options and clicking "Hard Mode: Off". -Wraith and Wexor
# Rapid Fire: By quickly tapping the left mouse button, you will with most automatics be able to fire faster than by just holding the firing button down. Doing this will, on the other have, make the recoil tougher than normally. But keep in mind that if the recoil becomes too rough, you can just switch to your other weapon with E, and then switch back to the weapon with the tough recoil, and it will be back to normal. -Johan
# Backup: If you're using a 1-hit-kill weapon such as the Desert Eagle or Hades Rifle, make sure that your second weapon is something a little bit more appropriate, like a high-powered automatic, to fall back on. -randomViolence
# Penetration: The X6 Flechette Rifle shoots through several enemies, making it good to use against mobs of enemies, and can be used as a Gauss weapon. -randomViolence