I have been playing this game alot solo, and I find that the most efficient way to play is with 2 mono decks rather than 1 mixed deck. Some may disagree but I find that the game gets easier and scales differently with two players (and more, I havent bothered with 3 or 4 players). I found that Spirit/Lore is a great combination. Spirit allows the player to demolish questing and get massive progress tokens, Lore can heal damage and neutralize monsters using forest snare. The two decks are great for self preservation. What combos work well based on play. I tried spirit/tactics but did not have much luck. Tactics seems to be the Rodney Dangerfield of decks, the one that gets no respect.
Best 2 mono decks to play with
There is a compilation of some of the more popular card combos, both for single sphere and dual sphere decks. you can find it here: boardgamegeek.com/geeklist/66685/lord-of-the-rings-combo-compendium
Based on the combos that compliment your playstyle, you can go from there.
Why not take it a step further and use two dual-sphere deck?
For example, you could have a Spirit (Major) / Leadership (Minor) deck and a Lore (Major) / Tactics (Minor) deck.
This way, you're able to use all the sphere's strength and cover all your weaknesses.
I'd suggest Eowyn / Dunhere / Gloin and Beravor / Denethor / Legolas as your heroes. This gives you reasonable starting threat (26-27, enough to deal with the Troll in scenario # 2).
I've been trying a mix between Spirit and Tactics. My hope is to use Eowyn with The Favor of the Lady attachments along with Lorien Guide allies to push the party quickly along through locations and the quest. Then use Thalin and Gimli from the Tactics to take care of the enemies we face. I have Thalin or allies act as defenders usually and give damage at times to Gimli on purpose because his power gives him damage bonuses based on how much damage he has. Then if you add an axe and armor, you can get up to do 12 points of damage per round. It's been a bit dicy using these combination for a solo deck and I'll try a few more, but it's what I have for now.