The way to easily 3 star that one is to use a palp lead empire team. Palp, Vader, RG, TFP, 5th plus a Phasma or Tarkin ally should do the trick (given they are geared of course). Rebels simply don't have the suvivablilty needed to last against 5 bosses, and don't have enough damage to kill them immediately (might get one before they get a turn but not all 5).