Avoid its lethal beam, it can do up to 1500~ hp of damage. Try to be at full hp at all times. Mages should use mana shield and stay far and diagonal from it while shooting SDs, Icicles or Ice Strike, or if high enough level, you can block them diagonally by using Ice Strike.
High level mages can make massive amounts of experience by hunting Hellfire Fighters in the Dark Path of Demon Forge. Just lure 3-5 and kill them with Avalanche runes, and remember to always check your mana. Don't forget to bring lots of supplies.