In which chapter does this quest come? In general i would try to combine an avalanche harmonics attack with all fury form attacks in one turn. Make sure you have enogh SP for it. You could also consider to use some of the Ether items. Does he geneate fully or only a fixed amount of HP?

Pretty late of me but it's probably the three zombie sidequest, my suggestion? Turn Rinne into a crit monster with regal gauntlets, the fairy that increases crit damage, equipping luck x2 effect, hit rate up 20% (because she has terrible hit rate as an axe user lol) equip the crit damage up 50% and the 3 symbol doubles crit rate effect, then rather than have Fluer (as crazy as that sounds) use another character because Glace sings a recovery song and the AI is a jerk about making the monsters be in the crossover point that increases the recovery by an absurd amount, like the difference of 400 base heal compared to 700ish, I'd suggest a tank built character goes in and breaks their mask/armour too so you're not wasting SP, all that should be more than enough to kill it on the first run, if fluer can't be taken out I suggest equipping her with maelstrom and maybe magic range +1 to keep her at a safe distance but still add to damage dood.