I don't remember if this boss heals or not, but I don't think so.

This boss does icewave right, the thing that ruins your tension and other support spells?

I would just have Angelo multiheal every round. You're bound to live if he does that.

Have Yangus repeatedly use Executioner again and again and nothing else. You will get critical hits going for 200 or so and that will add up seriously fast.

I would have Jessica/ Hero alternate Psyche Up, only going to 20 if the boss can nullify it (I say alternate so that way only one is ever nullified and both didn't waste a bunch of turns at the same time). I would psyche up Jessica and use either Twin Dragon Lash or if she has Kasizzle or whatever the hell the big Fireball that attacks single targets is called then go for that. I like Falcon Slash for Hero myself.

Angelo's MP should last long enough for you to win this way. If he starts to get low just make sure some characters have Magic Water on them.