If I recall, you have to kill a saber, put deathmark on Traya, then standalone with Han (make sure he has Tenacity Up from Chirrut). No idea if they coded stand alone in the Conquest boss though.
In general, I'm finding these boss fights a little lackluster. First off, there is no visible boss health bar. Second, your characters are all so powerful it's not like running the raid--I was not able to actually run Chirpatine like Chirpatine, for example. So I have no idea what's going on with Traya (haven't gotten there yet).