Fastest way I ever killed a troll: Abyssal razor, sneak up to the troll from behind, use the special attack (middle mouse button) - one-hitted the troll. Saw it in a video and wanted to see if it really was a thing. Yes. It is.

I prefer knives because you just can attack faster. Knife combined with a bone tower shield (which won't block ALL damage from a troll but a decent amount) has worked very well for me - better than bow and arrows (what can I say - once a tank, always a tank) ...