Even outside of multiboxing, when a mob gets a tank MC ability, it's usually meant as something to be avoided rather than dealt with. For example in Crypts, the Possessors have extremely low HP and take quite a long time to cast their MCs, with a very obvious 'channeling' graphic. It seems clear they're meant to be taken care of before the MC actually happens.

The controllers in Sethekk drop totems that act as the source of their MC, so you could make a macro to target the totem and frost shock to release it. Aside from that, if it's a timed MC, I just repeatedly cast chain heal on all my shammies, cross my fingers, and get ready to taunt as soon as it wears off.