Are you not able to sim those battles using a sim ticket?
If you can't currently, try putting in characters whose only goal is to survive and let your Ally character win the battle while your characters focus on not dying (or even bringing people back to life, if you have Old daka or Ewok Elder.
If you have an ally with Rey, that might be best since she can easily keep all your other characters alive through bonus protection and still win the battle herself. (She doesn't even have to have her ultimate.)
If you don't have an ally with Rey, there is a thread on the forums here specifically for people looking for GL allies. Tell people you're interested in Rey (JML would also work, but Rey is better for this because in theory an AoE attack could still get around JML's taunt and kill one of your toons).
People are good here. Someone will volunteer an ally code for Rey if you don't have one already.