Mace Skeletons are a type of Skeleton Chains that swing gigantic, spiked ball-and-chains over their heads. They are usually positioned in strategic locations and are a bit more resilient to damage than other Skeletons encountered in the same stages they appear in.
They perform their swings on a timed pattern, but besides of that, they don't have any other attacks, becoming merely roadblocks in the stage. Use hit-and-run tactics in order to defeat them, or if necessary, resort to ranged combat. On the other hand, a fully powered weapon may be enough to put them down in a single attack charge.