Also, just beat the "secret" Ultimate Choice difficulty. Still don't have a full grasp on the parallel bosses yet (and I don't have any other players to help me do the quad hamster splash), so for anyone in my position, here's the strategy that I've found useful so far:
Adeleine | Metaknight | The Three Mage-Sisters | Chef Kawasaki
Adeleine's guard has perfect invulnerability, and her healing ability has pretty low cooldown, so using her as leader allows you to just focus on adding support fire and keeping your team healthy while they do the heavy lifting. Another reason why her guard is nice is because of all the new bosses that are included in this mode -- so I've found that abusing her guard is a nice way to simply observe some of these bosses and memorize their attack patterns.
I was originally trying to use Metaknight as the leader so I could take full advantage of his moveset, but I found that Adeleine yielded stronger results. That being said, I haven't seen him get talked about at all, which is a shame because his new counter move is absolutely insane. He's not quite as safe as somebody like Marx, but I still feel like his combat versatility and damage output are both really high. Since he's a sword user, he can also interrupt some of Parallel Metaknight and Morpho Knight EX's attacks, which is kind of useful. So, that's why he's on this list.
Third and fourth picks are basically toss-ups, but The Three Mage-Sisters are a good elemental support if only because all of their elements are quickly accessible. They've also got pretty good damage output and iframes on a lot of their moves, so that's always a good thing. I think Marx or Magolor could have also been chosen instead -- again, just a toss-up.
Chef Kawasaki is basically just insurance policy, and something of a wildcard. Sometimes he'll just make some food, and other times he'll actually pull off an attack with it, but either way it's nice to have some of that support function in a CPU in case you're unable to provide it yourself.
All the game's bosses appear in their strongest variant forms, so this means you'll see Goldon and Silver etc. along with the Parallel Dimension bosses. Both the final boss and Morpho Knight have been amped up for this difficulty level as well. Morpho Knight EX isn't incredibly different, but his giant sword attacks have windboxes now, so be ready for that.
* Paint food at the start of the fight. If you're lucky, you might also be able to sneak in more food during the pinch animation.
* Make sure your allies are equipped with the proper element before the fight. In particular, definitely make sure that someone is able to melt Francisca's ice cubes, since you'll be rewarded food from that. Also make sure your party is prepped with the fire element and water/ice element for the final boss, because that will help with some of the shooting phases.
For the final fight: the shooting segments are a lot more intense, but you can still avoid a lot of attacks just by circle-strafing and tactically jumping. The shockwaves are a lot harder to dodge (some of them are in the air now), so you'll probably want to stay relatively far away so that you have time to anticipate them. Most of the weak spots are unlocked from the offset, so just focus on positioning yourself defensively while you fire away.
Final phase is pretty nuts, but as Adeleine you'll have plenty of opportunities to restore health to your team if you're paying attention to them. If you're not playing as Adeleine.... Yeah, I dunno, you're on your own. lol