Make a getaway power part of his package of abilities.
low levels, a ring of invisibility might suffice, middling levels a Cape of the Mountebank should work.
In either case, give the PCs a means to know that he might pull this kind of trick before they encounter him.
If they are clever enough to prepare and neutralize the enemy's "plan A" for escape, good for them, if not, let him get away without feeling bad about it.
Make sure to play up the look of horror in his eyes when the heroes open the combat attempting to sunder his cloak.
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You could also consider contingencied teleportation spells, such as Word of Recall, cast upon him by one (or multiple) allies or "backers" (not quite allies, but powerful entities to whom this barbarian of yours owes allegiance). It's an old trope, but not a bad one.
At lower levels you could have the players run away from him (aka, make him to strong for them so they have to run away.) If you do this make sure he has a reason not to follow them at that time. Reasons could be something he's protecting, or someone. Maybe the party has knocked a friend of him unconscious.
He could also have low-level minions that rush in to attack when you're close to defeating him, giving him the opportunity to make a tactical retreat.