Mighty Blow grants bonus XP for killing multiple enemies with one attack. Do channeled skills like Tempest Rush, Ray of Frost, and Rapid Fire count as a single attack for this purpose?
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.
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:
Sustained attacks seem to be exactly what the Massacre kill-streak bonus was designed for. Killing multiple enemies with de-facto different shots or attacks (even though they are of the same skill) and being rewarded with the Might Blow skill seems counter intuitive, since it's called Mighty Blow after all, and it does give some unfair advantage to players who regularly use these skills (although this doesn't really matter, it doesn't give a disadvantage to other classes) - but it still works.
To be more precise, in my tests I found that the only difference between Massacre and Mighty Blow seemed to be that all kills in Mighty Blow must result from one and the same attack of a certain skill, while Massacre can combine an arbitrary amount of skills. This results in sustained attacks of skills like Rapid Fire counting as one "blow".
Mighty blow requires you to kill at least six monsters in quick succession using a single skill.
It's very similar to the massacre bonus, except that massacre requires you to kill at least 10 enemies in quick succession using multiple skills.
Because it's based on kill timing and the use of a single skill it does not matter whether rapid-fire or channeled skills are implemented as a single attack or multiple attacks.