In Attack of the Mutant when Skipper defeats the Masked Mutant by tricking him into turning into acid - the Mutant could turn into anything solid, but if he turned into a liquid he would be locked in that form.
The villain himself gets one with his entire plan to capture Skipper. To elaborate, the Mutant takes the form of a young girl named Libby to get close to the kid, while his henchman(disguised as him) has captured a super hero. When Skipper finds out, he comes back to the lair, where "Libby" kills the villain, saving his life... before revealing herself as the mutant, explaining the ruse, then moving in for the kill. Up until then, very few villains in the series had displayed this much ruthlessness and cunning.
The episode provides examples of:
Skipper triumphantly mimics the Mutant's Evil Laugh upon defeating him.
