Syzygy I was aware the fight scaled, though having never known the specifics of that scaling until two days ago I was more confused by it than anything: my party was sufficiently experienced that I got the two-party battle with Bizarro/Rebirth Sephiroth, and after every turn the game asked me if I was “worried about the other members,” then offering a yes-or-no choice. I didn’t know what it really meant, and I said no every time because I wasn’t worried—my starting party was having no trouble. I didn’t understand to begin with what it wanted to incentivize in the party composition (and was resistant to looking at a guide during the fight, because I’d been staring at guides during the whole preceding dungeon (both times…)), so I stacked the first party with my strongest characters and never switched as they were never in any danger (which seems strange as Fandom’s description of the two-party fight makes it seem like switching parties is more or less necessary to succeed). One Slash-All from Cid’s Venus Gospel (with my Cid’s high MP) was enough to kill the head and limbs, which prevented the body from doing substantial damage to me. I never did any damage to the core (because I wasn’t switching parties?), meaning Seph never used Heartless Angel, meaning my party was never really in any danger. 2x Cut with Cloud’s Ultima Weapon ensured Seph’s heals didn’t amount to much. Safer Sephiroth only got to use Supernova once, and at that all my party members survived the attack.
This may be the wrong way to think about it, but as my average party level was closer to 68 than 53, I think I was on the upper bound of the two-party tier, and the fight was easier than if I had been bumped to the three-party fight.