My way to "cheese" Killavolt as a Beastmaster on high mayhem level:

Respec in a way that your pet gets maximum damage, and you get maximum survivability for yourself (high health regen, damage sharing, high pet bonuses). Dealing damage is not as important, this is more of war of artrition.

As a pet, pick Jabber with SMG and rocketlauncher. The rocketlauncher had extreme damage when it lands - use attack command to trigger it. I'm not sure what's going on under the hood, maybe it scales with enemies and with mayhem level?

As a action skill, pick Gamma Burst that will heal you, you can hide in the bubble and catch your breath.

Rest of the fight will be you trying to dodge attacks, and position Killavolt so he's facing his back to you pet. Then spam Attack command to slowly destroy him with Jabber's rockets.

It's not elegant or most efficient, but it works if you absolutely must.