When you are fighting the giant crab in chapter 5 then you probably know you have to shoot at the red tips on each leg, but when the monster comes crashing down and opens its mouth, shooting at it does absolutely nothing. What you are meant to do is get hand grenades (if you have none then look around the circle computer and you will find 2 herbs (green) and a hand grenade, DO NOT throw it at the beast. It also does nothing, go near the mouth when it is open after you have shot at the legs then it will say "press 'X' to throw" obviously you should press 'X' then Chris will throw the grenade in its mouth and the top shell will fall off. You must do this twice. If you have no grenades, bad luck you have to wait for the mosquito-like things (i call them Skeeters) and shoot them and they will drop a grenade sooner or later, then after you have done that twice, RUN! he will swipe without warning and it will be a one hit kill, no "dodge" chances at all, just run. When you have done all that, the easy part is getting your handgun (by the way the mosquitoes drop X20 bullets at a time, a good idea to pick em all up.) and shooting it once in the brain, it will be dead after that.