It took me probably 20-25 attempts to beat T7 with some Leader swaps along the way. My winning squad was Lvl 85 EP (L)(G10), Vader (G10), RG (G9), GMT (G9), and Magma (G8). I was too lazy to do much on the Mod swap front and didn't even have all of Magma's Mods to Lvl 15.
As stated by others the key is to hit R2 at every available opportunity...I didn't even care if someone else was one shot from death as R2 is just too deadly not to take down. For me the AoE of EP and Vader slowly sapped the Health of everyone else (Magma and GMT were pretty useless on the damage front). Yes Chewie will taunt, possibly even on turn 1, but just fight through it.
Best news is that compared to other characters R2 is actually not as ridiculous to Gear up.