Looking at Trolls individually vs units then they aren't as great as one would think. The AI doesn't use Trolls correctly. They are usually the first into battle plain and simple. This way amassing the required six hitpoints to bring trolls down is no great feat. However, I agree that siege weapons are the best antidote for Trolls. ONE DIRECT HIT and they are gone. This is different than with any melee or arrow unit. Not a single one of the latter can bring down a troll with one hit because of their 6 HP advantage. That is why I think siege engines are the best and fastest thing to bring down trolls, albeit that your engines can fire depending on the terrain, etc.