While I am cruising through Act IV I pause the game before each Champion / Rare pack that I need on the list to verify that it isn't checked off. Then after I kill the pack I pause the game and check the Achievement to verify that the check was made. After a handful of mobs not getting checked off...I feel comfortable saying that this must be bugged.
Before I continue "wasting" my time searching for these mobs...am I doing something wrong...or is this indeed bugged?
Just for reference:
1. I am by myself in the game so I am the only person dealing the damage.
2. I have my Merc sitting in town so they aren't dealing any "Killing Blows".
3. I have logged out and even exited the game to see if it needed to reset before I got credit.