Update: I'm calling this broken. The TA game leaderboard shows my progress correctly, so I have to believe that the achievement is not unlocking when it should. I've sent a message to the developers. I'll update this if I find out anything.
Since no one has earned this yet, I'll share my experience...
So far, I'm in Act 3 (Normal difficulty) and gotten 5 clean Gourgaz kills. Whereas the Giak and Drakkar achievements had running totals in the app, this doesn't seem to register. Therefore I don't know if it's glitched or if it simply doesn't pop until all 30 are killed.
Just a word of warning for now. The Vordak may be the same. I've killed two with no progress.