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1-Most important thing is remember to use your Piston whenever you can, to save ammo and to practice your aiming head shot ability.
2-Pick route that avoid boss fight if it's possible.
3-Penesis first fight: Use Shotgun to damage it and switch to Piston to destroy the missile.
4-To kill Liker, you just need 2-3 Piston on it head when it transform, but sometime you must hit it body to slow it down and take advance to collect ammo instead kill it immediately. Also best way to kill Liker is shot the explode barrel.
5-Running Zombie is very fast, use the Shotgun if it come too close. Remember never use Grenade Launcher on Zombie because if they're too close you can damaged yourself, sometime you can even hit some close objects like the street bulb or car and it's suicide.
6-Penesis second fight: Use the Grenade Launcher to damage it and switch to Piston to stop the tentacle.
7-When control Rick, always have the Assault Rift ready when enter a scene, quick shoot the Running Zombie, hit Liker body to cancel it transform and collect ammo then kill it with Piston head shot. The explode barrel also very helpful.
8-The Hunter is dangerous because it's very fast, but they're also easy to kill, head shot it with the Assault Rift before anything else.
9-When control Lara back, always have the Assault Rift ready when enter a scene, kill the Armored Zombie with Piston on it body because it's very slow and tough.
10-When meet Penesis on the bridge, choose "Try to resist him". That'll help you avoid the fight with Nicole later, she's really dangerous with her grenade.

Originally posted by Afftar Mudac:

There is a legal secret cheat. You can pause the game and quit to main menu if you feel unlucky in a current fight ;D The game will start from the begining of the last fight scene.