I finished the bonus tier with a Boba lead and 4 Troopers (zVeers, DT, Shore, Storm). I did the first two tiers with IG88 instead of Stormtrooper, but that didn't work in the bonus tier as IG88 and/or Shore usually got killed in the 4th wave and even if everyone made it to the end I still lost. My Boba is G12, the troopers are G9-11.
With that lineup you have to make sure Boba kills lots of enemies in early waves with the event ability. The best way is to whittle them down with trooper attacks and finish them with Boba's aoe. Shore and Storm make the team sufficiently robust. The last wave is over quickly if you enter with 20+ offense bonus stacks on Boba. Stormtrooper with omega on his basic is also very effective with the exposes from the event ability as has already been mentioned.
Bottom line is that also mixed faction squads can beat the event, even though they are a compromise. Troopers only under zVeers or Bounty Hunters only under zBossk are certainly the easiest way but my Starck isn't 7*ed yet and Snowtrooper not sufficiently geared, which is why I used Boba.