I got my 7* with Lando (L), STH, Biggs, K2SO and Leia. It took me quite a few tries too, I think I got lucky with RNG and swapping mods around.
I think the key is to go for Palp straight out, then with Lando all modded up, his AOE takes out the guards and keeps Vader and Palp low enough on health. I saved STH's taunt till about halfway through the match (until K2SO was gone) to help give my team a speed nudge.
I love EP, to me, he is one of the most iconic characters, and he's a tough old man. Although I'm still finding he's still no match for all the rebel meta's out there.
