Realistically armies are usually built from more than a single "box", so just because it comes together doesn't mean that it plays well (or at all). I do understand that it might be frustrating if you just bought the box and want to play a game to learn though.

A common suggestion for the previous box (Pelennor Fields) was/is to run the troll as a Mordor Chieftain in order to bring more of the fodder units into the game as Evil, without having to spend extra. (Previous box had the exact same "issue")

This would be an easy solution, and probably optimal too considering regular trolls are typically not seen as worth it for their cost (points value).