Initial thoughts anyway. Some of these need more analysis.
Rubicant, Milon, Valvalis, Cagnazzo (FF4) vs X, Zero, Cinnamon, Massimo (MMXCM): MMX can't really blitz Val due to spin, Cin/Massimo aren't that great. They can blitz Rubi somewhat effectively, but Val + the others is good enough seeing as at least one of the heavy hitters drops before this point.
Myria (BoF1) vs Randi, Popoi, Purim (SoM): Think the speed just overwhelms. Only one reviver doesn't cut it here.
Daos, Erim, Gades, Amon (Lufias) vs Profound Darkness (PS4): Lufia 2 voter. Any sinistrals that make it to the final form get FRIED by triple damage Megid.
Odd Eye, Geshp, Cameela, Zalbard (SF2) vs Ashera and Sephiran (FE10): Pretty high respect for the devils here, and they have the right style of offence to deal with Ashera (good mix of strong attacks to either defence).
Magus Three (FFX) vs Ghaleon, Xenobia, and Royce (Lunars): Magus three don't have enough damage? What crazy version of FFX is this. Sandy does something like 3-4PCHP hits by herself, and Cindy is excellent support and a death tank. Magus Three regularly take down much better superbosses than this.
Rofel Wodring, Vormav Tingel, Kletian Drowa (FFT) vs Maya Schrodinger, Todd Dukakis, Shady Thousand, Alfred Schroedinger (WA3): Too many of FFT team's tricks (status, equip breaking) don't work here. Okay, Kletian isn't spoiled, but there's a very real chance he dies to the combined damage of Shady and Alfred, because it's freaking Kletian.
Mewtwo (PKMN) vs Mario, Princess Toadstool, Mallow (SMRPG): SMRPG's ability to hit both defences makes a fight out of this, but it's not enough. After Barrier, Mewtwo's -lower- durability is 2x, and the SMRPG people struggle to one-round average, so Mewtwo's further buffs and speed put this away. I guess there's Sleepy Time hype or something, not too much faith in it myself, but maybe...
Red, Asellus, White Rose, TimeLord, Fuse (Saga) vs Xorn and Emelious (G3): Neph's strategy doesn't work too well because Xorn/Emelious are highly resistant to stat downs. I suppose if TL uses all 8 turns, he can get one of them reliably though. TL... should probably do just that, because otherwise Emelious will OHKO him. So TE the fuck out of Emelious, have the rest of the team blitz him down while Xorn isn't that scary... yeah, should work. Especially since the team has two revivers.