When facing megapeades don't try to shoot them because while trying to shoot them they wil try to squeeze the li out of you. So use a spear but first enter the unliminted spears code and try to hit them.
When Jack all you can do to the V-Rex is slow it down. The most efficient way to do this is with a spear of any kind. Shooting at it will enventually work, but using a spear is quicker. When you have to slow a V-Rex down, shoot at it until you can get a spear, throw the spear somewhere near it's head, and repeat the process.
When Kong, you can actually kill a V-Rex. To do this, simply hit the V-Rex as many times as possible. Once you hit it enough times, it will fall down. When this happens, you must go over to the V-rex and hit A to get on top of him. Then hit A as fast as you can. If there are multiple V-Rex opponents, do the same thing , but alternate your attacks.