When your at the heliport fighting that mutant freak, use A or B to spin. Then while youre spinning use your shotgun (make sure you save ammo). He takes 3 clips and in no time he'll evolve, then do the same thing until his dead.
Rocket launcher
Successfully complete the game on the normal difficulty setting with an "S" ranking and save the game when prompted. Load the saved file to begin a new game with the rocket launcher and infinite ammunition for it.
The ammo you want
Every time you kill the big guy from Resident Evil 2 he will usualy drop ammo. The way to get the best ammo is to kill him with certain types of ammo. Let me explain, if you kill him with the handgun you'll get shotgun ammo so you always get the next best ammo. So, from handgun - shotgun - flame rounds - acid rounds - grenade rounds - magnum ammo.