Well I did this at 62 with my 5 shamans.

Basically my setup for healing is I have lesser healing wave on focus bound to Shfit + Space. I also can do a 5X group heal if needed, but not needed for this fight.

Ok I like to set up at max range, pulling from max range you should have time to kill both adds before the trio reaches you, that makes it easier.

Then I just spam lightning bolt on the main, watching who has aggro, as soon as they get half cast lesser heal wave on him with all guys (makes it easy with just pressing shift +space), then go right back to dpsing with LB.

Using this strat I have never wiped to him, worst I did was lose one shaman.