I just did it with my G8 Phoenix squad (didn't use Sabine, see here https://swgoh.gg/u/koragaz/collection/?f=phoenix ). Tried it with the original bad mods they had and I got annihilated. Swapped over my arena mods and I did it first try. Couple of things that helped me:
1) Make sure all your skills are out of cooldown at the end of the 3rd phase. Go into 4th phase prepared!
2) Zeb is important! Try to 7* him and use as much potency on him as you can. Thrawn has really high tenacity, but I managed to stagger/stun him a couple of times and that helped a lot.
3) Use as many damage mods (critical damage, chance, offense) as you can on Ezra. He'll be able to dispose of adds really fast.
4) Careful with your taunts. I used a combo of Hera's rez and Kanan's taunt at the start. On my first failed try with the bad mods, the rez didn't work when a fractured Kanan died (rez buff applied after fracture, might be a bug? The buff icon was there, so it should've worked). Remember, dead Kanan means no counter attacks.
5) Kill order: Deathtrooper->Healers->Stromtrooper->Thrawn.
That was my experience, hope it helps!