Multi headed is his buff which causes him to hit additional target with his melee attacks, that is consntant. What do you need to care about are Flashing Fangs - its a short cast time then series of brutal blows, you gotta have your Shield of Righteous (is it the pally mitigation mechanic now?
) up, else you might die...
http://www.wowhead.com/spell=227514/flashing-fangs
The problem is, that you can easily get melee hit from Multi Headed, Flashing fangs and another hit from Multiheaded in short span which kills you if you have no mitigation up for it.