I've just got the ered mithrin cycle and decided to set up a thematic deck around the creature and beornings type.
Considering the heroes, despite starting from a high threat (32) it can be lowered using woodmen clearing and/or Gandalf's ability. Secret paths and Radagast cunning also proved useful for a successful quest. The necklace of girion and radagast's staff should be attached to Radagast to help in questing and paying for the creatures. Attach leather boots to Beravor so that she can be sent to the quest and ready when a location is revealed and keep using her card-draw. Notice that she will need to quest in the early game. Grimbeorn should take a bow of yew to boost his attack and his main purpose is facing enemies to get rid of encounter cards when drawn as shadows. Use beorn's rage, faint and outmached to make the most of him.
Henamarth will help you prepare for the round, especially to design your strategy to face treacheries. Use Gandalf's search to rearrange the cards coming into your hand in following rounds. Beorn's hospitalitat will come in handy uf your heroes and allies have taken in too much damage.
Regarding allies, the hound will add to the quest, defend and can be discarded to 'heal' damage resulting from undefended attacks. Wild stallion should go to skin-changers to improve their numbers, especially when attacking and defending. Wait 'till their about to die to discard them and be replaced by powerful allies, such as bears and beorn especially. Getting beorn together with the bow of yew and a wild stallion will take you to the upperhand when it comes to combat. He also comboes great with outmached and beorn's rage. Once you get a powerful beorning, wild bears can also absorb damage resulting from defending against powerful enemies, such as trolls. On top of that, beorn can be retaken from the discard pile by means of another skin-changer. The eagles will mainly help to quest and provide 2 progress, so don't worry much about using them for defense purpose as long as you can get them flowing using Gandalf's search and gwaihir's debt.
Obviously, the effects of treacheries are the main drawbacks the deck faces. The main way to get rid of them is by means of confronting enemies and discarding encounter cards as shadows. Luck when drawing, though, will play a rellevant role, so using Henamarth, Gandalf's search, secret paths and Radagast's cunning are key when planning the move for the round.
I hope you'll have fun using the deck and, although it can't guarantee 100% success, you'll enjoy an adventurous wandering across rhovanion. It's the first deck I've designed myself, so I'm sure you'll find ways to improve it.