One frame remains for breaking damage attenuation...

And it's Zephyr! Specifically, Funnel Clouds seem to resolve before attenuation is applied. I've been successfully oneshotting acolytes with my heavy slam Melee Duplicate Magistar across both rounds.

I haven't tested if this a property of the blast interaction with tornadoes, or if non-blast heavy slams (which are rare) also nuke enemies before attenuation applies. I also haven't tested if this is specifically a result of Melee Duplicate shenanigans with tornadoes or if, say, Doughty builds such as the Ruvox would also work.

Zephyr stays winning!