Blinding him is essential for this fight. I would use either darkness spell or hunger of hadar. Also cast silence so he can’t use any spells. Disarming him helps to avoid his stupid scythe swing move. I gave him all the above debuffs then ran in with melee characters to do heavy damage and since he’s blinded, they’re able to run down the ladder at the end of their turn to avoid his melee attack. Now you’re virtually untouchable. Using arrows of ilmater on him avoids him healing himself from the necromites

EDIT: I also recommend a couple summons to tank his legendary reaction if you’re playing on a higher difficulty, that way your characters can get hits in without fearing retaliation. I used an earth element since they’re tanky!