First, go to your cheat page and turn on Invicibility, Infinite Ammo and All guns if you have them, but Invincibility is esential. Start any mission on any difficulty. For this trick you need grenades. Get out a grenade and then hold Z for a few seconds while standing next to some enemies. When the grenade explodes, massive damage is done to all the enemies around you.
*Hint* This little bit of suicide works good on Jaws in the Aztec level, and in level 2,Facility, when the alarm starts to go off and all the enemies chase you.