"If you've been into every achievement of this game and spent tons of hours on them, you might find that this must be the hardest achievement of this game. Because it gives us feel of much randomness.
Beaten the story and gone down over 100 floors in Shadow Vault, and then unfortunately you find that you've killed no more than 10 champion trolls. While if you're motivated enough to go down the Vault again and again you'll find that there won't even be one promised champion troll in every 10 floor.
Sad is that you are not even able to imagine how many hours you'll need to unlock this achievement, because you have no clue about where the champion troll will appear.
Nevertheless, this does not mean it can't be unlocked. Thanks to the Runic Games, they still keep the console available to players. Please allow me to show the magic here:
Open your console (Shift + ~); Input "champion troll"; Enter. WARNING: Doing this will label your character as a cheater and the shared stash will become inaccessible!
You tell me what happens after that. Good luck! "
Or one could simply go back to the Molten Prison (levels 21-24) and clear it multiple times. One shouldn't have to do this too many times, since you'll get quite a few trolls already if you do all the "play-through" achievements (defeat Ordrak on *insert difficulty*)