While you may find skeletons in Tassel Town, don't worry about them. Instead, this is more of a stage-specific trophy that should be doable on: Hypno Baron's Castle, stage 1.
You'll find plenty of skeletons running around inside the castle itself. Switch to Elephant Dance and dash into them. You might get some damage taken as well, but you have to do this several times to kill the required amount. Don't worry about running out of skeletons, as they respawn quite frequently.