I think turn order and Ahsoka/GK are the key. I was struggling with all G12 except G11 3PO and R7 GK. Upgraded Ahsoka to R2 and that was what was able to push me past. It seems as though you need to be able to kill B2 before their team takes a turn. Turn order for me was:
Shaak Ti (Call Padme to assist on B2 to give everyone prot up)
Padme (use stun/assists on B2)
Ahsoka (special on B2)
3PO (mass assist on B2 for the hard kill)
GG won’t take a turn until his allies take 10 turns, so any kills you make before that won’t trigger his bonus turn (or rather it does, but he sits at 100%TM without taking a turn). I think on the one where I beat it I was able to get both B2 and B1 before GG took any turns.
