I haven't needed to do anything special with any of the generals so far. I don't know why people seem to be having trouble. It takes time, but it's not particularly difficult.
Make sure you're at max level and are going in fully charged, including meditating at the hot spring.
Then just go in and keep hammering away at them with everything you've got.
Use the invisibility potion early on, and after getting down a bit, the health and magic potions (they are slow recharge types, remember).
Use your area-of-effect spells to deal with the side demons. Keep your main focus on the boss.
If you get "accosted", don't just struggle out of it right away. Use it as an opportunity to let your spells recharge . The allure potion might help you there.
Whenever your Celestian form is available, use it, especially the time stop action. This is your main advantage over them.
Eventually the demons stop recharging and you can finally defeat them.