The only time I'll risk blowing my big EM+NS+CL combo on a mage is if there are TWO of them and I really need to roll the dice and try to burn one down before the arcane explosion spam starts. I've killed a lot of mages with a Lightning Bolt + EM+NS+CL combo... basically landing all the DPS at once before they realize they need to ice block. I have had a fair amount of success by launching my initial LB volley against a paladin in order to force him to bubble, then immediately popping one of the mages.

I agree with everyone else though, prioritize classes that don't have immunity options, or at least force the immunity options to be used up early. Teams that are doing the target=targettarget can be a pain, but if you keep switching, you can use that against them so that they're healing the wrong/old target...