It was warriors with their helicopter thingy at 70, and now it's well geared frost mages. If I don't take them out at range, it's free corpse run time for me. I haven't leveled my 70 mage up yet, so I don't know what it is, but when they freeze you and they have something that stuns you, then blizzard. My suggestion is a friendly insta-cast lavaburst volley to say hello before they get near you.