It's heavily RNG dependent. On my 60th or 70th tried I barely made it with Vader 1v1 Chewie. They did take like 10 turns before me too.
My team consisted of G10 Palps, G10 Vader, G9 TFP, G8 RG, G7 SnowT. I tried taking out R2 first but it was extremely pointless. He probably has the same amount of HP as the Rancor... Get rid of the hard hitters, Leia, Han and Luke. If you still have all of your 5 guys standing you might have a very low amount of chance to finish them off. If your Palps or Vader gets targeted by Han on his first turn, retreat. There is no point in fighting them at that point. If you can't stun more than 2 people with your first Palps Aoe - it's 99% certain that you will lose. Sometimes even if you stun them all(without R2 obviously), you could still lose. So it's really down to RNG.