Auto skills only last for 3 turns until they wear off, so they are not good at all at boss fights. Ameno-sagiri has two turns, so he'll dish out -dyne spells and God's Judgement, which cuts your HP in half. When he uses Quake, he'll always follow up with Fog of Confusion. At that point, you cannot hurt him while Fog of Confusion is out. He then buffs himself, so that's where Trumpeter/Debilitate really comes in handy because once he buffs himself and drops the fog, he'll use that eye laser, and you want to be blocking during that attack.

For the most part, just keep whacking him with Mind Charge + -dyne spells. If you have Chie or Kanji, get Power Chargin'. And heals.