I think it needs to be a regular troll and I don't know if spawning one would work. KR has placed one of each creature in certain areas to ensure you can get them all no matter what level you are when you do this quest. Now, remember that you need to kill the troll within the 30 second duration of the spell or it won't work...
Anyway you can find the KR-placed troll here:
Spoiler
Near Salt Marsh Cavern entrance.
btw, you should probably have posted this in the spoilers section, not general discussion.