This is one of the most consistent and sturdy decks I've managed to put together. It works well in solo (It cruised through most of the Dreamchaser cycle, only really struggling with the immediate assault of The Havens Burn) and shines as a "fill-in-the-gap" deck in multi-player. We need a questing deck? I'm on it. You need me to defend and kill things? Treebeard loves to smash orcs. And who doesn't love to see an Elrond-blessed Warden of Healing hit the table? Masochists, that's who.
The key to the deck's engine is Elven-light. It's going to give you an extra card alongside the capacity to transfer Arwen's resource to Elrond each turn if/when you've nothing from Spirit in your hand. Absolutely mulligan for it. If that doesn't work, Heed the Dream, Daeron's Runes, and Gather Information are all there to help you find it.
Your next problem to solve is action advantage. Treebeard can do just about anything well but choosing how to use him early on is a real conundrum. An early Lembas, Unexpected Courage, Light of Valinor (for Elrond), Late Adventurer, or even Henamarth Riversong (in solo) can help ease those decisions. Once you've got enough breathing room to do so, this deck's power ramps up with every Wellinghall Preserver you can get into play. They're costly and you'll have to survive their first round in play without their help, which is why they aren't an automatic buy when you're under duress, but once they get going they open up so many options for you. Buy them asap!
Lastly, you start off with a high threat and at first glance there isn't much to help there. Elrond's Counsel is often enough by itself, I've found, but the real key is to remember that you can recycle a used Favor of the Valar with Erebor Hammersmith. If you face an encounter deck dedicated to raising your threat you'll want one or two more copies of Favor of the Valar but against most quests you have enough here. If you're playing solo the real challenge may be what to do with a tough early enemy. That hill troll or that Black Rider is engaging you on turn one and you'll just have to deal with it. That's the price you pay for employing some of Middle Earth's most powerful heroes. Good luck!
I made a couple changes to this that are reflected in a newer version. I dropped both copies of Late Adventurer for a third copy of both Light of Valinor and Henamarth Riversong, since I found both to be better first round options.
In multi-player I'd drop down to only one copy of Henamarth Riversong and replace the others with whatever you need from the sideboard.