This achievement may still be bugged, but even if it isn’t, it really isn’t worth it.
IMPORTANT NOTE! concerning the Natural Selection achievement that I will place here: Please comment if you know how and I will update. Note – I could not figure out how to do a 2v2 Skirmish against the AI, nor did I see how to adjust the point value to play with other point values than 1200 such as 1500 or 1650.
If you want to test out fleet ideas against the AI (at 1200 point value), select “BATTLE” on the main menu, select Skirmish 1v1 on the left (right side is ranked), create a custom classic fleet and name it to save it, choose your skills, then just click Launch, and since you did not invite a player, it will pop up a screen where you will choose the AI faction you want to fight and whether it gets more or less points than you.