The best way to kill them is to get some alts and trap it. If any friends are on have them get their best bow or sling (bows preferred and really good arrows) and attempt to kill the troll. Having high constitution is a must or you are most likely going to die. You will usually need good armor to survive.
Trolls give troll bones and a troll skull; these items are some of the rarest items in the game. Trolls have about 550 ua.
Trolls Have the Following -Known- stats at level one mine:
It is generally seen that these stats increase by .5 per mine level.