@CoastalJames without bossk (or jango, aurra or embo), cad bane is your best option. I tried with ig88 g11 and cad bane g9, and they both die instantly most runs... But on those where they don't, cad bane can stun and that is amazingly useful.
I eventually got it with boba g12, greedo g11, zam g10, cad and dengar g9. Took many many attempts until rngesus showed me mercy. On that run I killed Chewie, and could then stun and kill Han. With those 2 gone, it's "easy". Even so, only greedo (barely) and boba (after 2 revives) survived the battle. Zam died in the opening salvo.