Skeletons, how do they work?

Solution 1:

From the Dungeon Defenders Wiki:

Each Dark Elf Mage may spawn up to three Skeletons. As long as the parent Dark Elf Mage is alive, it will revive any Skeletons that are "killed". Since the Dark Elf Mages tend to stay out of reach of defenses, players should prioritize killing the Dark Elf Mage over any Skeletons.

A defeated Skeleton that is awaiting revival will appear on the map with a slightly darker icon.

In the Halloween Spooktacular! challenge, Skeletons spawn independently like any other monster, but with three "lives". A few seconds after being defeated, they will revive themselves, requiring players to kill them three times.