Note that necro can heal both pet and themselves pretty good, and since you can insta-summon pet you can use up to 4 tanks for a difficult fight. You rarely need more healing. You can also switch to chloro and elementalist for better healing and tanking if you really need to. It's pretty easy to beat elite rift bosses with two mages, I think.

The only hard part is to finish the timed elite stages in time. It's hard to kill all of them before the time runs out. Is that easier with a tank + healer? I have yet to try it, but I have the characters created for when I do...