Complete the game three times and unlock all three endings. Regina can start a game with the grenade gun with infinite ammo. Go to the Parts Storage room that contains the Core Parts. Follow the small hallway to the end, then turn left to find the grenade gun upgrade.
If you don't want to use the rocket launcher, during the final fight take out your shotgun. Arm the custom parts to it. Once done, blast away at his head. He should be defeated in about five to ten shots.
Since the dinos are unable to pass through a shutter, use them to your advantage. Close each shutter after you pass them. This way, if a dino appears in that specific area, you can not be attacked (unless, of course, you are on the same side as the dino).
When you must decide between Gail's way or Rick's way for the final time, choose Rick's way and use the scanner to find dr. Kirk. Go after him. This leads to the fourth and final ending. Everyone will still be alive (except Cooper) and you do not have the third energy data disc.
There are some areas that you will only pass through once or twice in the entire game. Killing the dinos in these areas doesn't matter. More importantly, it is a waste of ammo. Use the weakest tranquilizer dart on it/them, and move on. This way, the bullets/shells/slugs/grenades are available when you need them. Once you get infninite grenades (by completing the game three times under the normal difficulty setting), you can kill those dinos, without wasting ammo.