Go into the room with the C4 and crouch under all the red lazers.Then pick up the gun,ammo, and THEN C4. After that LET the invisible creature attack you, when he gets on you press B really fast and then the R trigger. If you did this right you will have done a critical hit and only wasted 1 ammo!(not to mention the fact that you didnt miss and blow the entire room to kingdom kong.
Anyway i hope this helped!