It’s borked. I looked at the code in the editor and it checks for each unit built if it’s a merc, scv, medivac, or not. The problem is it doesn’t specify WHICH player it’s checking. So whenever the AI zerg decides to make a unit, the achievement gets locked off.
Hold off on this one until it’s fixed. Literally impossible as-is. “Band of brothers” on gates of hell has a similar mechanic, but that one works, provided you don’t use spider mines (I think)