I disagree, trolls are to be feared and that is really nice in TATW, they are suppose to be a challange! I think stat wise they are perfect and in single player the AI usually does not field more than 3 units in a full stack, at least that's what I observed. ~So the only balance I see useful here would be to make them more expensive to purchase and to upkeep, EDIT: And maybe slow them down a bit, they are awfully fast on the battle field!
Playing as High Elves right now and to be honest I had mayor problems at first as well. My full stack army of mainly archers was slaughterd by a full stack of misty mountain snaga/spearmen with 2 and a half unit of trolls. I concentrated my fire on the trolls which was a big mistake. Slaughter the cheap masses first so you have more moverability and can free up units to pin down the trolls, since Archers alone are simply useless. Arrows don't do much damage to them. You have to pin them down first, then move archers around and use flaming missiles. Yes, you will hit your own troops as well, but the heck you fighting trolls!! This way you take them down! First the masses then the big boys and you win!