This is how you kill a troll at (relatively) low level.
Step #1) Beat the troll into hamburger by normal means (e.i. subdual damage 4 or more than thier current h.p.)
Step #2) Take a torch and perform a coup de gras action. Specifically, tell the DM that you are going to hold the torch against the trolls brain pan or other sensitive surface.
NOTE TO NEW DUNGEON CRAWLERS: Just because you have magical lightsources does not mean that you shouldn't be carrying around torches. Fire has infinite uses.
Step #3) Your torch does 1d2 fire damage to the troll. Because it is a weapon, and you are performing a coup de gras action, you do double damage (2d4) AND equally importantly the Troll must make a Fort save (DC 10 + damage dealt). Since the average troll has a Fort save bonus of +11, there is only between a 5% and 15% chance that the Troll will recieve critical damage on any given round. However, since the troll is recieving damage roughly as fast as it can regenerate it, it is only a matter of time before you kill it one way or the other.
If you have a flaming weapon, this works even better.
Now, if your DM is really sick (like me), he lets you kill trolls like this a few times until it becomes routine and then he runs you up against trolls with a few levels of cleric and both Endure Elements (Fire) and Endure Elements (Acid) cast on them. Suddenly trolls become feared opponents again.