Prebuff as many things as possible and spam Power Extraction until 35%, then Spin2win.
Slot Merciless one or two packs before the boss, to have stacks when starting it, like I did on the Acuity setup for exemple.
You should make sure to have an Acuity setup with Destro ult for whenever you plan on ulting. I made it with Kazpian here, but if you don't trust your Alkosh buddy's uptime (or you don't have it), use Balorgh and 4x pieces Lokkestiiz.