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.

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.

While Ash is within the zone of her ultimate, your ult is unable to affect her, so save yours for another time.

In a straight DPS fight, Vivian does more damage than Zhin. If you have Deflector Shield up, you're likely to win the 1v1. Be wary of his ultimate and Whirl, as both will ignore your shield and the latter can place him behind you.