If you struggle against the BIG RED SKULL, let it be known that each time you hurt it, it brings up a shield of spikes that kills the mice which touched it instantly. You must thus retry, and each time let your past incarnations do the damage till they died.
For instance, on your first attempt, you might land a hit on the first "weak" phase of the SKULL, then die. On your second attempt, wait out the first "weak" phase - your past incarnation should hit it for you. Then, on its second "weak" phase, you can hit it again. Repeat in a third attempt and only hit it at its third "weak" phase, and - tadaaa! It should explode into a blast of EPIC PROPORTIONS.