This is the best I've come up with so far for a viable mono-Spirit deck. Don't expect to beat many quests on Challenge mode, but you should have a decent shot at most quests on Adventure mode.

Opening Hand

You're looking for a Hobbit Pipe in your opening hand (goes on Tom), as you will absolutely need whatever card draw you can get. The pipe will net you a card every time you trigger Glorfindel's ability, every time you play Galadriel's Handmaiden, and for any threat reduction events you play (e.g. A Watchful Peace). Other good cards to see include Star Brooch, Sting (your only real combat booster), and possibly Fair and Perilous or Seen and Unseen if the scenario starts with some nasty enemies in play (sub in a second copy of the latter for scenarios where there are two big uglies in play at the beginning).

Early Game

First Star Brooch goes on Glorfindel, as does Sting. With Fair and Perilous, he's your go-to for any enemy you need to one-shot, so get him set up as early as possible. The second copy, if it shows up, goes on Éowyn. She can also take an Unexpected Courage. The trick is to find the best place to stall for a couple of turns, since generally speaking you won't be worrying about threat too much. If the first location is good for this, then stay there. Otherwise, progress as quickly as possible to the next one. With the high of this deck, that shouldn't be an issue for questing-based objectives, so feel free to deal with lingering enemies by running away!

Mid Game

Once you've found a reasonably safe place to stall, get out some allies, particularly Sarn Ford Sentry and Rhovanion Outrider -- they're the most likely to stick around a bit. Build up some resources, too, so that you can drop down Tom Bombadil and Gandalf on the final location as you race to the finish line.

Late Game

This deck doesn't have a great late game strategy. Basically, try to survive and go after the final objective as fast and hard as possible. Don't be afraid to lose a hero or two in the process -- there's not a lot of raw to work with, so sacrifices will have to be made. Good luck!

Miscellaneous Notes

• Ride Them Down! is an excellent card when played at the right time. You really need to know the quest you're playing in order to use this card to its maximum advantage, so take note of where there are big objectives coming up that coincide with Sauron having some enemies on the table.
• Sterner than Steel is often not needed, but for some quests it can win you the game in combination with Ride Them Down! -- so sub it in as needed.
• Gilthoniel! A Elbereth! is hard to time, but can be a game changer to get rid of big, nasty enemies with block or a boatload of hitpoints. Great for Hill Trolls, for example (sub in another one on that quest!)
• Tom's ability will save a lot of if used properly, so pay attention to timing it. Note that Sauron is smart enough to attack with a small enemy if you've got Tom's ability active, though.
• Unexpected Courage is surprisingly not that useful here, but can be a back-up to Star Brooch for Éowyn. I don't recommend it as much for Glorfindel unless he's got Sting attached.
• If you really need more , you can try Narya for late game. Don't play it early, as without healing, Gandalf will probably die on you before you get to the final objective.