This is the most difficult Scenario we have, and thous the most driven by luck...I think this because there are some cards who can enerve you a lot in soe moments or being useless to stop you in others.

But after so many game in single player, I think succeding alone it's imposible...any advice?

I've beaten it a couple times with two players, and once with one player.

The solo win was with a deck made up almost entirely of Rohan Allies, and showcasing Eowyn. The key is to get massive questing and blast through the scenario as quickly as possible. Spirit has a major advantage because it can handle the locations that you are not allowed to travel to, clearing the staging area and allowing you to continue questing.

If your questing is high enough, you shouldn't have too much trouble with threat, which means you can fight the harder enemies at your leasure. I strongly recommend having 3 copies of Eomer in your deck. That guy allows you to use everything you have to quest, and then throws himself in the way of your enemies to bring your forces back to fight.

Good luck!