Have any of you tried fighting Lavos and Queen Zeal with...
...JUST Magus?
I know, I know... you can't have just one party member during these battles, at least not without modding the game. But Magus is very capable on his own (he "doesn't play well with others," as I like to say), and of course he even >!takes on Lavos alone in his own ending. 😬 (Good luck, buddy...)!<
Heck, he gets to override the boss theme against his own mom. I don't think this can happen with the final battle, but ya gotta admit it's satisfying to see him succeed against Lavos alone.
Although you can't remove the other characters from the party, you can bring them in at 1HP so they'll die immediately in the first phase. You'll have to just let them stand there in subsequent phases, though. You can wait for them to get knocked out, or just have Magus attack while he uses them as meat shields. 🥩
What do you get for this? NOTHING! Except maybe satisfaction that you did it. ❤️ This will probably require at least a couple of NG+ restarts.
Extra credit:
Use only Magus from the moment you get him. Have the other characters stand inactive in battle. Don't finish the Death Peak quest (or if you do, use Magus/Robo/Ayla -- all three are low dialog priority in the last scene, so you may get some dialog you haven't seen before).