Fire Giant Cleric in Yaga-Shura's keep.
Likes to cast Regeneration, followed by Summon Fire Elemental, Aerial Servant and then spams Hold on your party.
Has near 100% Magic Resistance; it took four castings of Lower Resistance (at 25% a pop) before he stopped shrugging off my spells.
Retreat to the doorway. He and his Giant companions can't follow you through, but the Fell Cats and elementals will. Keep him pinned down with your archer to make his spells fail until you can lower his MR enough to put a DOT like Insect Plague on him.
Awarded 40500 exp upon death (normal difficulty, no mods).