There are a lot of attachments that you can put on Grimbeorn the Old because you want probably readying, stat boosting and ressource generation on him. So it's hard to decide which restricted attachments he should get.
Armored Destrier and Rohan Warhorse are great choses for Grimbeorn to ready him in every deck, but with Elfhelm on the table they become even stronger. Now they also give him +1 attack which brings him (counting the defense lowering) to the magic attacking number of 6, so he can quick strike most weaker enemies. You can boost him up even further with Dúnedain Marks. Snowbourn Scouts are there to throw them under the bus against everything to big.
Many allies in the deck are mainly there to bring these attachments out faster, namely Westfold Horse-breeder, Galadriel, Weather Hills Watchman (which can also help to bring some healing in form of Dúnedain Remedy if necessary) and Sneak Attack/Gandalf which is mosty used for card draw. It's often hard to get the right cards into play wihout playing lore, but with these tricks and the Spirit staples Elven-light and Ancient Mathom the deck runs very smoothly.
With combat handled by our skin-changing friend, the rest of the deck is mostly there to muster questing strength. Éowyn gives a strong basis right from the start and Ethir Swordsman can help us to rich very high numbers.
Sideboard: The Day's Rising and Desperate Defense are there if you don't have enough core sets for all these copies of Steward of Gondor and Unexpected Courage or another player decided to be the Steward-guy. Sailor of Lune can replace Ethir Swordsman in quests with direct damage effects and Well Warned helps in quests with a lot of threat raises.