Listen to Fingolfin26. Get Deathmark on one of your people and keep them alivr however you can--if you're using Galactic Republic you can take no-damage turns to get it on Kenobi, he's a beefy boi so he'll be easy to keep alive. Unfortunately he's also very slow so he's hard to get to go before any enemies do damage. Then spread damage around so Anakin can finish them off with one AoE.
If you were able to do all the other feats, you'll only need to get the three kills on T6 for max reward.
For folks with Rey as their only GL, I was able to do it with her, GM Yoda, Revan, and Anakin; each one has an AoE. I let the enemy Rey get Deathmark, then used Revan to mark Chewbacca since he's got a bunch of hp ans I could spread damage between the two of them. Eventually Finn taunts, ignore him and get the other two down to yellow, then youe Rey uses her murder special to instantly defeat Finn, and the AoE portion should take out the other two.
