You fight the Grizzled Alpha at the end of the Abandoned Sawmill in the Blightbogs, during act 3. The sawmill is either up towards the North-East top of the Blightbog map, or the very north.
This is a storyline achievement.
Just a warning, took me three separate kills of him to unlock the achievement. Had a similar issue with Cacklespit. Good luck.
Here is where it was for me on two of my play-throughs