Straight up... WTF BLIZZARD !!!
The bug whereby certain rares/champions (no point listing them here, there are countless threads doing that already) either will not spawn or spawn but dont get credit, thus preventing this achievments is STILL HAPPENING, and STILL HAS NOT BEEN ACKNOWLEDGED
Serioiusly Blizzard, are you reading this Bug report Forum ?? this problem has been KNOWN... FOR MONTHS !!!
All we want at this stage is ACKNOWLEDGMENT... but not even that.