I thought I'd be sneaky and try getting the custom achievements playing "the fastest map possible". Just beat 7 insane AIs in FFA, but didn't get the "Insane FFA" achievement... why not? Does it have to be done on a Blizzard map?
I believe that it does have to be a blizzard sponsored map. Easiest way to get it in this case is to play on Lava Flow as terran. As soon as the game starts, load your scvs into your command center and fly to one of the islands. Get an engineering bay pretty soon, and start to pump out turrents along the edges, and go strait to battle cruisers. Soon there will only be one AI left, and you should have close to 20 cruisers, so you should be able to steamroll.
I developed my own way to do this. First I tried using just one base on abyss and walling off with loads of tanks and turrets but I always ran out of money repairing my tanks. So next I tried walling off at the wide choke after the natural. Just build 2 bunkers and rax there and then build 3 planetary fortresses to seal the choke ASAP. Build a bunch of tanks and turrets and no one can get in. The AIs will pretty much destroy each other and then you can just build a nice air force and destroy whats left. Took me about an hour but this method should work faster than that as I played very timidly.
Oh and I set all AIs to terran.
Like Austen said, take one of the islands and mass battlecruisers with turrets surrounding island
Only use sweeps to scout players, don't send our units to scout or else you'll draw attention
Then, get one medivac and send scvs to the other island to mine vespene safely
Get Yamato, and pwnt, use orbital command for detection or get a raven
Deny Expos and demolish tech (so they can't get vikings, corruptors, or voids/phoenix/carriers depending on the race)