For me the best place, and where I did it is in the Edron's troll cave:
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I just checked the amount and here is the full cave:
Killed Monsters:
- 8x bear
- 1x frost troll
- 4x rat
- 124x troll
- 26x troll champion
These are the 3 floors of the cave:
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Although there are not many troll champion, you can also do the bestiary of them in here, since they appear in bigger amounts in the lower caves