This other question has several good tips for how to get the Insane FFA achievement. I tried one of the strategies and it worked well.
However I'm trying to do the ones where you have to have Insane AI allies (3v3, 4v4) and for some reason my allies do nothing. I've tried 4-5 matches and it pretty much goes down the same way each time. My allies build up a decent force very quickly (no defense at all though), and they don't do any thing. The computer will then team up one on of them, by the time reinforcements get there the ally is already destroyed.
I thought maybe my allies were just waiting for me to initiate an attack, but when I tried that it didn't go so well. They stayed at the bases and my entire force was lost.
Any tips to make this easier? Or should I just stick with the FFA tactic and go hide on an island and build up an Battlecruiser army. (Of course, then it becomes 1v3 or 1v4 since my allies will all be dead).