To easily kill the bosses for levels 2, 4, 6 and 7, toss a grenade at them. When they begin flashing after being hit, pause the game. As long as the game is paused, the bosses will keep suffering damage as they flash. If you wait long enough, one grenade can be used to kill the bosses.

After you beat the large mutant that is blocking your way, and the Plutonium Boss shows up, try to trap him in the top right corner of the screen and blast him with all the grenades and gunshots you can fire. He may hit you once, twice, or three times, but you will hit him many more times than that. This tip works since almost 100 percent of the time he is in the corner. He cannot aim his whip diagonally, making it almost perfectly safe to defeat him without getting killed. It may take some practice.