I'm lvl 57 and have been stuck with low arena rankings until I recently put together a new line up. My new team is a work in progress but it is as follows.
5* Phasma, only IV atm.
4* stormtrooper Han VII
4* Kylo Ren almost VII
4* jedi consular VII
4* luminara almost VII and 5*
Han is the key in the line up simply due to the taunt that lasts 3 rounds, luminara and JC keep his hp up for the taunt duration. But the real synergy comes from the massive turn meter advantage produced from Han taking hits. This compounds with the advantage gained from Phasma. Some fights I can get in two turns from all my heroes plus additional bonus attacks from using Phasma as my leader. All before the enemy has a chance to respond.
Han will usually die shortly after his taunt ends or sometimes on the last turn of his taunt. But by that time I have generated enough free swings to the out key targets and win the fight.