This one is pretty well-known. Throw a grenade at a boss and when it scores a hit, pause the game. Leave it paused several seconds and then unpause. The boss will be defeated instantly. This will only work on certain bosses, like the brain, the two giant insects, and the two giant frogs. It can work on the crab, but it's very difficult to do. Also, be careful not to hit pause while Jason is taking a hit because he'll continue to take damage, too, and when you unpause, he'll die.
Taken with permission from Flying Omelette.