They can leave the dragon in, but there needs to be a trigger for the Kith'rak to take dragon and leave the fight to Baretha. So, for example, suddenly, drow spring up with goblins from the west path to "save the Absolute's Chosen, though the PCs have no idea why they attack the dragon, etc. They have a ton with them. Voss then says, "Baretha. Split their skulls open. Qudenos and I will handle these istiks." Then the dragon and rider fight off the Drow assault while you fight the Gith.
