From a siege prespective, I'd say go with ballistae and, once they close to the walls, dropped heavy rubbble and boiling oil.
From a combat prespective, longbows or crossbows (we know Legolas can put an arrow into them) or if you have to deal with one closing with you a good pike. Plus a pike is useful because if a troll charges you can set the pike and it will skewer iteslf, effectively turning it's natural strength against it.
The trolls main advantages are strength and reach. Long ranged attacks, like bows, negate the strength advantages because strength is useless if it can't be applied (and trolls don't seem to use any ranged weapons, thankfully.) Reach can be negated by both ranged attacks and using a weapon that extends your natural reach.
Edit - Phantom looks like great minds think alike.
You beat me to the post while I was typing.