What is the criteria for "killed X creatures with one blow"?
From my experience so far, you get Mighty Blow if you kill 6 or more enemies under X seconds.
I have not scientifically measured it, but I think the X is between 0.5 and 1.
It seems completely independent as to the source of the kill - a single skill activation, a channelled skill activation, multiple hits, traps - as long as it's >6 kills in <X seconds, you get the Mighty Blow bonus.