Do you have to melee them?
I noticed that the Ash Bow is called "Troll Slayer". Maybe that's a hint?
Checked my old save. Bow skill is "feeble" and cutting is "weak" (now modest). Found a way to grind, though. You can save on the first level, so I just kill several trolls, return back and save on the first level stairs. Fire spell is good for killing trolls, but there are giant army of them on level 3. I can't even use invisibility spell because they block narrow passages.
And with just increased to "fair" strength I'm finally not overburdened.
This is my own character after having cleared the Goblin Cave and found the Tome of the Ancients:
I've spent all excess money on herbs so far, but I see Power Control has increased quite a bit, so I guess it's safer to cast Heal now.
I'll also buy an Ash Bow if I know I'm about to enter a Troll infested dungeon.
Although relying on magic may be safer, since in my experience one casting uses far less "action points" than loosing one arrow (which rarely do much damage anyway). So I'm using bow outdoors, magic indoors and melee against weaklings and critically hurt enemies.