Kuwaga

This answer might be a little late, but anyway,
Do you have the Longicolnis spear equipped? You need it to break through his shield. Another tip is to kill two of his dark minions each turn so he will keep casting his summoning thing every time. As you do that, just keep using Atropos/Clotho/Lachesis and your summoning spells against him and you should be able to kill him easy.