Supreme Commander
41 Achievements
1,000
Project X
Build 100 experimental units.
40
How to unlock the Project X achievement in Supreme Commander - Definitive Guide
While going for the Economy of Grand Scale: Mass achievement you can go for this one after you leave your xbox on. once you get back on and you know you have the achievement: Economy of Grand Scale: Mass just destroy most of you Mass makers and power generators, then start making the cheapest Experiment unit. Make sure you have plenty of Tech 3 ACU's to build the experimental units and have the settings on fastest.
1. Start a skirmish match and make sure enemies are not a threat.
2. Build lots and lots of Mass Fabricators and Tier 3 Power Generators on your way to 100M Mass.
3. Have ~10+ engineers/ACU/SCU's build strips of 10 Experimentals, 10 times.
4. Delete experimentals as needed..
5. Save the game to get the achievement! Continue game at your leisure to get 100M Mass eventually once you max your population of Fabricators and Generators.
Long version:
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.