There are a couple ways to use Trance Terra on Atma. A debuff-SPR + dual-cast Ultima should be able to take out 80-90% of Atma's health bar depending on how strong your friend TT is (~1,000+ mag) and Kefka's Last Footstep (deshell may also work, but obviously much riskier); two decent TT can kill Atma on first turn with DC Ultima. If your other party members can make up the remaining difference, you should be able to take out Atma on the first turn. The risk is if you don't kill him the first turn, and get his HP below 50% Atma can ruin your team. To counter this you can use Setzer's cursed card to paralyze him; two turns of dc Ultima should be enough with the right TT.
This is the first Mog Event where I prefer the 4* over the 5* friend.
I'm using Gau, Terra, Kefka, Setzer, Orlandu and Friend Setzer.
2 Setzer Red Card chaining with Orlandu Ruination kills Atma Weapon 1 turn.
If i bring a TT instead, there's a chance Atma wipes my team on 2nd turn :/
Second method is to use Magical Activation the first turn followed by Chaos Wave, which will also take out around 80-90% of Atma's health. As Chaos Wave chains very easy you can usually also kill him off with a strong Setzer. If you keep Atma's health over 50% you can survive whatever he dishes out the first turn; or you can just use Setzer's Cursed card to paralyze him.
I've been able to consistently clear ELT using TT, Setzer, and one half-leveled non-geared Kefka with Last Footstep. The rest of the units being unleveled bonus units.