Quick tips and first two level codes.
Play the practice mode to get used to the controls.
To kill the beasties, hide in the shadows (pressing up), and only
fire at them when they are reloading their gun.
Don't stand on the small metals plates that are scattered about
the floor, as these will reveal a big gun from the ceiling !!
Simply jump over them.
Level 2 : [FBWC]
Level 3 : [QP7R]