Diablo – Do pets count towards “per attack” and “on hit”

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.

It's a thorns effect. Every time you're hit by a melee attack, the offending servant of darkness will take X damage as a rebuke for daring to strike your most august personage.

In fact, if you look at the detailed breakdown of your defenses in your character sheet, it's even listed as 'Thorns' there.