I'm gonna be honest: 7 Star challenge is all about smoothing out RNG damage spikes. Easiest way is to get Barriss as leader. If that is not an option, go with Luminara as leader and have plenty of time and sanity. It will take a few hours to get the right allies and have yoda not attack too much consecutively.
My 1st hand experience: QGJ lead 30mins no luck, Luminara 30 mins no luck, Bariss done in 20 minutes. My setup was: Bariss (L), Lumi, QGJ, JC, Eeth.