My Rebel team strolled through to Round 8 on Military Might I. It then took me four attempts to beat Round 8 but I finally did so, unlocking Tier II in the process. I still get to Round 8 with rel...
You might want someone who can apply tenacity up or cleanse your team of the debuffs. Chirrut or Rex can help a lot for that. Tier 2 heads up is balanced more towards level 85 and Gear 10 or higher. I know that second part you might not want to hear, but it is the case with the other events like the Sith and First Order assault battles.