After 15+ attempts and 4 different party set-ups, I finally did it. The fight took a grueling 70 minutes and consumables in the excess of 100k GP. It was fun as hell, though, and definitely one of the most difficult fights I've ever attempted in an RPG. The strategy itself wasn't all that complicated, but actually staying alive throughout the entirety of the fight without any stupid mistakes was challenging. Celes/Setzer healed and supported the party with rerise/float. I occasionally spun bars with Setzer when I had the time, which somehow led to him hiding behind the edge of the screen after jumping off of Palidor. Shadow and Cyan's defense ignoring damage with Throw and Dispatch were some of the best ways I could feasibly outpace Atma's regen (especially in phase 2). Shadow also focused on keeping Image up on himself and Cyan during the second phase, so they could attack without getting decked in return.
Well, that's what everyone should've been doing, but there was a ton of item-flinging in between. At some points, I went several turns per character without even getting to attack lol. Anyway, special thanks go to Cyan and his 454 HP + 55 Stamina, which somehow allowed him to survive both Flare and Meteor 100% of the time with <15 HP remaining, leading to him dying the least amount of times by far.
ELs were as follows:
Celes: 4 Siren & 2 Seraph (+4 Spd/Mag & + 60HP/30MP)
Cyan: 6 Kirin (180 HP & 6 Stam)
Setzer: 6 Seraph (180HP/90MP)
Shadow: Nothing lol
Stacking HP/MP seems kind of necessary at these levels if you want to survive more than one hit. The only reason I don't have 6 Seraph levels on Celes is that I spent those first 4 before getting Seraph (which I forgot existed .-.)
I'm sure someone else has done this at even lower levels, so I'd love to know your strategy, and how mine could've been improved upon.