This is one of the few achievements you should not expect to get during the campaign or normal multiplayer progress. Time consuming but can be done on Skirmish on a large map vs. the AI. An easy method is to get your economy going and many Tech 3 mass extractors then take 10 or so Tech 3 engineers and queue up 100 experimental units of your choice on any map of your choice. The fastest to build is the Cybran's Monkeylord. Come back in two hours or so, save and quit if you get the achievement. If you don't, keep building. *Experimentals can only be built by a Tech 3 level Commander, Tech 3 engineers, or Support Commanders and can be built by navigating the click wheel to the "X" on the Tier selection menu.*
Note that you should do this after (or in conjunction with) the King of the Hill achievement since Experimental units are used to win those games.