Contrary to popular belief mages are can be pretty easy. You will always get a charge in on these guys. When you charge use battle shout or mortal strike them or whatever before they blink. After they blink they will always cast poly. When you see that they start casting you use blood rage! you will get free rage and get insta healed when in polymorph sweet huh?
If poly breaks early don't bother trying to intercept since you will most likely fail and be stuck in zerker stance for that nasty fire blast.
When poly breaks from dmg use the blazing emblem trinket if you have and intercept the mage. Now its important that you hamstring him before he blinks. You should have enough rage to mortal strike the bastard to before he blinks again. After this it should be enough with a critt and execute to kill him.