There are 3 ways to beat the Demon Generals. Use either.
1. You can forcefully defeat them by transforming into a Celestian whenever your health is low.
2. Complete Selena's events to get the succubus clothing set and unlock the skills that drain the General's MP. Most generals are powerless without their magic.
3. Summon a Noxtar before battle for help against the fight. You need to complete Noctriel's events for that.