Definitely one of the hardest fights in the mod, Atma Weapon is really worthy of being a "final boss" for the first half of the game. You will not roll over this guy, especially not at the anticipated level of around 20. A big part of the fight is making sure Shadow is leveled up and usable since he'll be stuck in your party no matter what (unless you ignore him, but then have fun fighting Atma with three).
Expect characters to die and be ready for it. Expect Mind Blast to derail somebody. It's going to happen even if you prepare for it with Amulets and the like (there's just one Ribbon, so you'd need Gau and/or Ghost Ring shenanigans to get more blanket immunity). The fight is won or lost based on whether you can avoid getting stuck in a recovery loop and whether you can continue doing damage to Atma consistently throughout the fight. Leave him be too long and he'll Regen way too much health back.
My strategy was to lean on Berserked Mog with an Ice Rod, hitting his weakness with a critical spellcast for immense damage due to stacking multipliers. Nearly everyone else was set up to support Mog's damage and keep the party in good shape. Celes and Terra are bulky, Shadow has good buffs, so that wasn't too hard all things considered. Mind Blast is, as ever, an annoyance, and with Mog uncontrollable it's important that not everybody die, but that aside things went about as well as I could've expected them to go.
Note that halfway into the fight Atma Weapon "powers up" right before he uses Flare Star. This gives him some hidden buffs, including Safe/Shell. That can be removed with Dispel, but I found myself confused because Mog was still doing about half the damage. Turns out this is because he ran out of MP... the Ice Rod will only cast a critical spell if you have enough MP to activate the auto-critical, so Mog had simply run out of juice. In the future I'd probably remember to pop a Tincture on him with Shadow or something and things would've gone even faster toward the end. That is something to consider when you have one character doing all your Atma offense.
