After several weeks of messing around. I've managed to break the 50% mark for a single attempt in P1 AAT Heroic.
The team consists of Teebo(L), Cassian, Jyn, TFP and Barriss. Barriss has a zeta but I don't think it makes a huge difference for this phase.
It is a fairly tedious process with a "count to 7" requirement. Basically you want to hit GG with TMR every 7th time he takes damage. Buff block needs to be on GG all the time and on the adds as much as possible.
Barriss is a champ here, she takes an insane amount of turns once the debuffs build up. This is how she keeps the team alive. She gains 10% TM per debuff dispelled. You can see above that everyone is topped off on Health.
Mod wise, nothing extremely crazy:
TFP has about 80ish potency with 216% CD and about 60% CC, 250+ speed
Teebo has about 95% potency, 200+ speed
Jyn has about 30% potency and 265 speed
Barris has health primaries and about 220 speed
Cassian has about 220 speed and 110% potency
Credit to Maraxus, Trill, Bennet and Pez for the brainstorms and ideas.
P.S. I think a full solo could be possible but that would need extremely good RNG and a lot of time. This attempt took 40 minutes.
Bistan could also replace Jyn, here is a screenshot of a 40%+ attempt:
I think Cassian could be replaced but he provides 3 important contributions: Speed Down, Buff block (this one is more important) and the ability to execute a null damage ability every other turn. B2 may be a substitute but I have not tried that set up.