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Is Harold Westraven overpowered?

Last week I received my Demons of Darmas preconstructed deck and decided to test it against a gauntlet of suggested decks.

After several plays, I went to the conclusion that as long as Harold's adversary deck too much relies on allies, it is almost an auto win for Harold's player.

Hunter's Mark combined to Harvest Soul will tear opponent's allies to pieces, with no hope for him/her to get them back, thus nullifying any allies-based strategy. Throw in a Fire Archer/Stormwind Sniper or a Flash Archer with Secret Door (for Harvest Soul recursion) for good measure and Harold's opponent will be only left with its eyes to weep.

Since there is no way to react to Hunter's Mark during Harold's player turn, the only remaining option lays in cancelling Harvest Soul upon it being played; and I am not sure such a card exists.

So, I am wondering if I am right when asking if Harold Westraven is overpowered or if I missed something in my analysis. Otherwise, what would you suggest as a countermeasure then?