It only works in the battle arena and presumably guild battles, and those events are the ones where individual units can be eliminated. I was thinking that it might just outright oneshot a defending unit, which would be amazing, but I suspect that it just does extra damage (lame).
If so, it would only have an advantage over double/triple/quad attack if you're poisoned.
(for those of you who don't know, if you're poisoned when your quad attack procs, you take the poison damage each swing, for a total of 4 poison ticks)