The Grim Reaper tries to overwhelm you with summons. Pretty dicey fight for my party setup considering I had no good AoE options at the time.
What helped was shifting my Wizard to 2nd position along with giving her the robe that boosts staff physical damage, enabling her to kill some of the skeletons without needing to dip into her spirit resource pool, as skeletons are weak to bludgeoning damage.