Extremely powerful hero. He is now in my "power deck" as in the deck you pull out when a scenario is just ridiculously hard. I've actually gotten to where I don't use him unless I run into those types of scenarios because with the right cards he becomes quite broken.
Not only is his ability action efficient (defend and attack), but it also breaks certain enemies/shadow card chains. There are some enemies who do things like return to staging area after they attack. The timing of his ability is such that he can counter-attack before said enemy can take its escape action. Brilliant. It also breaks nasty shadow card effects like "makes an additional attack after this one".
Now here's the ultimate OP combo: Steward of Gondor - Grants tons of resources and the Gondor trait (this is important). Wealth of Gondor - Free card for extra resources, you'll need it. Gondorian Fire - Turns those tons of resources into tons of attack, which sticks around for the entire phase. Gondorian Shield- Two additional shields. Rohan Warhorse - Re-readies after he kills an enemy.
And lets not forget the thematic card Beorn's Rage, which will also greatly help him early on. The real trick is just making sure he gets plenty of resources.
Yes, it takes time to get all this going. But it's not like hes a slouch until then and obviously there will be other things at play. The point is within a few turns you'l be clearing the table of trolls and enemy captains with ease anywhere on the table. This is really useful if you are in a fellowship with a combat-deficient player when a boss type enemy bounces to the first player at some point in the round. Because he's sentinel and de-facto ranged he can defend and attack for anyone.
Now, with all that said. I find it frustrating that he has no synergy with beorning allies. Frankly, the two are in conflict with each-other. Beornings are relatively expensive allies and yes, I know about beorning-skin changer's cost-saving ability. Grimbeorn's ability is resource intense, and he's the type of hero that begs for a stack of attachments.The other thing is that Grimbeorn is so combat efficient that you don't necessarily need beorning allies.