For the final couple of battles you may want to consider Uranus. She has Light Veil which blocks attacks from the furthest range...which the last boss always sits at (hint hint!). Oh, Pluto has the same move I think as well, so that would fit your style!
Never used Jupiter, most of the game I ran Mars, Neptune and Chronos. Neptune cause I enjoyed the health boost, haha. But I swapped Chronos for Uranus and that was it for me.
Haha, I did the same thing with triangle! For the first half of the game, which caused some unnecessary stress.
Yup, I would buff and then throw everything you've got. So long as you out damage the heal you'll be good. I recall Phantom having too much health, you just might wipe it in a single full attack. And don't fret berserking your demon and running out of ...chain...energy....demon stuff, your Paladin taking hits for the team and you guys attacking should be enough to keep the berserk going.
I agree about the whistle, but that and Force Guard, coupled with Dark Veil (from Pluto) will make the last boss manageable; rather than frustrating. And consider it this way, with those types of buffs in place, they're wasting one of their actions and you still have four more from your team! Not a bad trade :)
Let me know how it goes!
