The fight is shit, it's something like a need hits/ougi/hits/ougi/DPS race for each 20%, his stacks change effect each 20%. I had hrunt, and luci/bubs in summons, I brought friend luci.

You have a 5 turn timer for each 20%, it might be 4 turns actually since you need him to do his 80/60/40% omen for it to swap characters and remove the debuff. I'm not sure if the 20% swaps characters but it didn't remove the debuff, I killed him within the 2 remaining debuff turns because his stacks went away . There's also omen RNG for each 20%, for the hits phase he has 4 debuffs/4 ougis, and in the ougi phase there's a 4 damage skills/can't remember the other one, and the last phase was 10 debuffs/5 ougis. One of the omens gives him refresh which if you fail you might as well reset since it needs 2 dispels iirc and gives him back 1%HP so he won't proc the phase change omen.

His dmg cap in the hits phase is like 15hits? It's a lot at any rate but you also can't have too many hits because and when he's in ougi phase he has a repel and starts at 5 stacks so if you don't have 3 dispels/6 debuffs you will take a lot of damage, if you go with 6 debuffs you still take a full turn of repel damage because it resets at the end of the turn, you can try 6 dispels instead so that reduces stacks before all your autos. Also in the last 20% you only get 1-2 skill uses for each character