This trick will work on levels 2, 4, 6 and 7.
Toss a grenade at the Boss.
Pause game play after the Boss begins to flash.
The Boss will accumulate damage while the game is paused.
A single grenade may be used kill the Boss with this technique.
Note: This trick only works on the bosses for levels 2, 4, 6, and 7.
During the boss battle, quickly throw grenades at the boss as fast as you can, he will begin to flash. Press pause while he's flashing and wait 25 seconds, unpause the game and the boss will explode. Easy as that.