When Vivian's ult is up, combo your Fire Spit and Salvo on her. The AoE damage will seriously hurt her and her ult drones, if not outright kill. If you can't get close enough, poke her from a distance.
Yagorath is the largest target in the game. You can shoot your Fire Spit and let it hit her directly for a significant damage boost. If you combine it with the Fusillade talent, your rockets will be able to overpower her natural damage reduction.
Unless recently killed, you can usually count on Luna to be somewhere near the fight, and for Io to be standing near her. Use your Shock Pulse on Luna for a free bounce between them. This might not apply if Io is using the Sacrifice talent.