i found crushto hc a twat of a boss with my 5x affliction warlocks team.
You have to interrupt his yell (so i had to switch out 3 of my voids for fellpups for starters!) and he jumps on random players and knockbacks etc...

i beat him once out of about 10 tries. first attempt i was i613 and that was some serious ass-handage
it wasnt until about i617 that i beat him and even then i couldnt easily repeat it as 2 tank pets just dont live long enough and he always came after my locks.
Like you say its easy to be out of range for pet healing and also my locks would get stuck behind things sometimes.

tldr: always interrupt his yell.