The best way I've found to deal with them is the drop nova, let them take that hit, followed by magma. 5x magma will quickly eat a mage fast. Shock if you can (the 3 frost, 2 earth is my method) and instead of chain healing (because interrupts on it suck and it's cast time is too long) I have a button bound to each shammy will target himself and LHW. I'll let the magmas eat away the mage(s) and worry about whatever else is dpsing me. Since as a caster you only need to be 180 degrees facing a target, I'll also turn 3 of my shammys around the other way and mash my shock button so I'll guarantee to get the mage as he circles in and out.

If there's more than one mage (or if at least one of them is fire with dragons breath, i'll bloodlust and chain heal. I personally find blizzard more annoying because of the chill effect.