Godlike
Persephone (Wild ARMs 5) vs Mewtwo (Pokémon): Gets off a limit immediately. Being somewhat faster than a Sentinel ends really badly by my views on how they work.
Dark Knight (Bravely Default) vs Arceus (Pokémon): Arceus can resist dark, but is powerless against a decent bit of Minus Strike spam. But... it'll take four Minus Strikes to kill a full-HP Arceus (each does 31.5%). If he opens with Cosmic Power and has a dark resistance set, the most a Dark Nebula can do is 9% of Arceus' HP, which is enough to kill if things set up perfectly... but they won't. Rage is the bigger concern, since Rage+Minus Strike x3 will... actually probably kill. Of course that can't happen until turn 3. So forget Cosmic Power, Arceus just wants to kill in three turns without letting Dark Knight set up perfectly. This... actually he should be able to do, his damage is magic so he can do plenty of it, and is much faster than DK so probably has a 4-3.
Heavy
Tiki (Fire Emblem Series) vs Oerba dia Vanille (Final Fantasy XIII): No reason to assume Snowfire isn't right.
Edgar Figaro (Final Fantasy VI) vs Alfina (Grandia III): Blocks Edgar's stuff, the reverse isn't true.
Bacchus D-79 (Star Ocean: The Last Hope) vs Billy Lee Black (Xenogears): Yikes. Assuming Billy can block stun, this could well be an endless fight? Billy has a 5HKO here, so even Haste won't get him past Emergency Repairs, but Bacchus has a low 2HKO which Billy can haste-heal away. The issue, of course, is resources... Bacchus' don't really run out (he has Convert), Billy's do. It's also posisble Bacchus could put Billy away with critical hits in rush mode. Billy's only hope is to store up a combo but even those only do 68% to Bacchus' durability... which can't even combine on a doubleturn with a normal DB for a KO.
Middle
Lightning Farron (Final Fantasy XIII) vs Ranger (Bravely Default): Lightning manages average HP? That's really surprising. gj Hope? (Doesn't matter here.) EDIT: On second thought... yeah Ranger's method of heal-locking doesn't work against Lightning, which is kinda sad since it doesn't take much damage for 4-ATB healing combos to not be good enough.
The Jackal (Bravely Default) vs la Grenouille de mort (Breath of Fire II): Jean doesn't beat many ID-immune Middles, and I'm not at all convinced Jackal is a Middle.
Light
Archer (Final Fantasy Tactics) vs Amalthea (The Legend of Heroes: Trails in the Sky): When in doubt, assume a Trails boss sucks.
Cyan Garamonde (Final Fantasy VI) vs Freelancer (Bravely Default): Freelancer blocking ID is really slow and Quadra Slice is ITD, so I wouldn't be surprised if Cyan can just punch through with damage. Or stop yeah sure. Or infinite Empowerer + more damage sure.
Pikachu (Pokémon) vs Rubella (Child of Light): This really needs a stat topic. Guess is Rubella, the game's damage curve doesn't feel -that- wide to me, but if it's wider than I think Pikachu may have a shot.
Noel Chandler (Star Ocean: The Second Story) vs Luke fon Fabre (Tales of the Abyss): Build up to a mystic arte or something, I dunno. Definitely disagree with considering regen, though! If we're taking PCs at points that aren't endgame now, I'm sure Marcus will be overjoyed...