I've only managed to beat two bosses so far. Vustrasz was beaten due to sick synergy between Frost Lich Jaina and Wax Elemental, which lets me constantly heal off damage from his hero power. Azari was beaten as Warlock, this time with Wax Elemental buffed by Captured Flag allowing me to freely trade off against nearly everything he puts out.
Wax Elemental be real good, yo. Especially if you N'zoth to get a whole crowd of them.