Thing, The Cheats

To get Witley on your team when you reach the dome at the very
start where you see Witley before you take a step throw a blue
grenade outside into the snow then the cutscene will appear and
Witley will be standing there instead of running.  Take your
bloodtest and use it on him.  Give him a weapon and you now
have Witley on your team.

More of a tip than a cheat. To avoid wasting ammo, and possibly getting killed/losing heath always close doors behind you when you have the option.

For example, on the level where you have to kill all the walkers, a walker will enter from the front door if you do not shut it.

Always shut the door of the room you walk into, if you have the chance. For example, on the level where you have to kill all the walkers, a walker will enter from the front door if you do not shut it.

Under the warehouse through a door is your first real boss. As soon as the cut scene has ended keep yourself to the left of the room keeping your back against the door. There is a tentacle that pops out of the wall on the right. Shot gun the hell out of the boss then use the flame-thrower on it until it collapses on the floor.
You will have to shoot the little spider things as your killing the boss because there is a hatching bubble just the other side of the boxes your standing next to on the left of the room.