The fight starts with 4 (lightning) skeletons, and 1 (fire) Bloodbones. After some of the enemies have been defeated, 2 more (ice) Bloodbones will appear in the center. 4 (dark) Cursed Bones will appear later on in all 4 corners of the map.
All enemies have a big weakness to strike attacks, with some decent resistances to everything else. But they don't really have high HP, so it's still easily doable with just about any units.