For some reason, bosses in P1 can be a little glitchy if you oneshot them, especially if they have a second phase or defeat dialogue. I’ve seen someone straight up softlock the game by oneshotting Kandori's first phase with a Bufudyne stone. Idk if this can happen in other versions of the game.
Let me make an update in the comments, with this weird... weird glitch that I discovered changing the playing field for my lil' Pacifist challenge idea I had going. If killing the boss with a reflect that does well over 3000 damage actually skips the tentacles part, then that means that little section of endless grinding for Tetrakarn and Makarakarn is slightly eliminated. However, I still believe it's impossible simply because the eyeball does zero damage. Though if I am able to find a second, even weirder, glitch within Thanatos's battle... then who knows? It may be possible to beat Snow Queen as a pacifist.