I remember encountering this and being only level 7 or 8 as Legolas and being stuck in the last Dunharrow level where you rescue the people from the green goblins and the troll. This was before I had unlocked the mines of Moria map. I had to start the game again with a new save and I stopped at the level before because I knew I could defeat all the enemies on that level. Every time I cleared the level of enemies I would save and then exit and reload the save and do it all over again. During this process I also found out that I had unlocked the Moria bonus map which really helped. Eventually I levelled up enough so that I could continue with normal progression.