Kill one of the unique enemies listed in the Never Seen That Before Challenge.

While in game, press Start, go to Challenges, then Triumps and scroll down to the Never Seen That Before Challenge for a full list of the Elite names to look out for. The rather long list shows off the many special Elites that are generally a bit more powerful than their lesser Elite counterparts, and can only be found (by chance) in a set location. It has been my experience that Drury Brown is the most common of the lot, appearing in one of the three tombs you need to search through for the Skeleton Kings crown in in the first half of Act I. Although, chances are you will get at least one of these guys over the course of your time spent getting the other achievements (probably quite a few on the list, actually).

As a side note, if you are going for the Staff of Herding to enter Whimsyshire (Cow Level), you will need to farm items, one of which is dropped by Chiltara. Chances are you will get this achievement naturally, but if not you can get two birds with one stone by killing Chiltara in Act III.