Do channeled skills count as one attack?

Solution 1:

You can get a Mighty Blow when you use a channeled skill, but only if you kill enough monsters fast enough. The fact that you use a channeled skill to kill enough monsters does not qualify for Mighty Blow unless you kill them quickly enough.

I'm pretty sure that, in general, Mighty Blow does not care at all on how you got the kills - from a single channeled skill, from an AoE attack, from a combination of mines and 2 different skills - it only cares about 6+ kills in a short enough timespan (about a second). So whether a skill is channeled or not has no impact here.

But either way I was able to 100% verify that Mighty Blow is obtainable from channeled skills, but only when you kill the monsters quickly enough.

Solution 2:

Mighty Blow bonuses trigger off of sustained attacks so long as several monsters die within a very short span of time. The easiest way to test this is on the first Act IV boss, who spawns a ridiculous amount of minions - sustained attacks such as the Wizard's Disintegrate can and will activate Mighty Blow, but only measure across a brief period of time - usually about two or three 'ticks' of the ability, though it may differ depending on which is in use.

My computer's too terrible for video evidence, but here's a screenshot of Mighty Blow triggering from just Disintegrate on the aforementioned boss: