Ambush, chain lightning, typhoon, thunderstorm, lava burst etc. A 25x multiboxer can oneshot with each one of these.
Assuming a starter gear (from BR) 25x multiboxer with 20 DPSs and an enemy 1200+ resilience DK these skills hit starting from around 1600 normally, and from around 2300 as a crit. With a 7% effective crit chance (20% mitigated by 13% from resilience) you have almost 33k of instant damage.
This is considering:
- a very decent resilience
- a DK with magic damage taken reduction
- a 5550 gs multiboxer
- I used the weakest skills i've listed, in fact a lava burst hits around 3k normally (non crit damage).
So yes, probably already a 5.8 25x multiboxer can oneshot anybody with each of these skills.
Also to consider is the fact that bis geared chars usually have pve offset, and I'm pretty sure that1200+ resilience is an overestimation, especially in not-arena settings.