Been trying to get the achievement for 'defeat 7 insane AI in a free for all' and it turned out that all the other AI were eliminated by each other, but the last one was just freaking ridiculous. Given that it's AI and not a player the bag of tricks is going to be limited, can you share any experience you have at taking down the insane AI or any exploitable weaknesses you've noticed in how it plays? For instance maybe one race tends to be a little easier to beat than another?
Take this answer with a grain of salt, because I have not tried it myself yet. But I've read elsewhere (mainly the official SC2 forums) that your best bet is to:
1) Turtle up and wall off best you can, while waiting for the AI's to kill each other (which sounds like you already do)
2) Pick a map with island expansions which you can aggressively expand to and easily defend.
Not sure what your preferred race is, but the aggressive island expansion obviously favors a Terran selection (as does walling off).
Hopefully someone else can come along and offer some advice based on actual experience :)
UPDATE: A SC2 forum post talking about this. They suggest Extinction or Lava Flow for the map. Once suggests going Toss and massing Colossi while making all your enemies Zerg. Another suggests mass Battle Cruisers from the islands.
There's a specific guide for this achievement in gamefaqs. To summarize, what you do is pick a certain map called Extinction, pick protoss(and your enemies should be all zerg), then basically, turtle your way to victory. Block your opening with some early photon cannons, then you will have to expand your base in the 2 islands in this map. That way you can turtle in peace(you will eventually lose your main base).
On the islands, protect your bases with cannons and then start churning out carriers. By the time you get to finish building all your carriers(this will be your main attack force obviously) there will be 3 or 2 AI left for you to pick off(since it is a free for all)
more in depth explanation here: http://www.gamefaqs.com/pc/939643-starcraft-ii-wings-of-liberty/faqs/60593
(haven't tried this yet since i'm still scared to try insane, but two of my friends confirmed this works - and they have the orlan achievement to prove it)
This worked for me and is in fact easy for anyone to do:
1) Select the map "Lava Flows" and play as terran
2) All your oponents should be Zerg
3) When the game starts, only harvest minerals and build only 10 SCVs.
4) When you have 400 minerals, build a second Command Center.
5) When the second Command Center finishes, load both of them with 5 SCVs and fly one to the island in top left corner and the second one to the island in bottom left corner.
6) Harvest like crazy and build ONLY battlecruisers.
7) Take out the last AI that survives (the right time to attack is when you have about 150/200 supply, the AIs will probably attack you at that moment anyway, but with 12+ BCs they stand no chance, trust me).
Easiest way to do this is to start game on the map Extinction. Set your self to Terran and all your Opponents to Protoss. The reason you set the Comps to Prostoss is most of their attack will be Ground base with the Occasional Void Ray. Lift of to the Little Island and tech to Cruisers. When you do your intial lift off only 5 of the 6 SCVs will be able to go with you. Take the 6th and run him to high yield area. after you get some resources use the 6th to build an Expo and lift it off to the high ground expo where the watch towers are. dont invest too much into this expo cause it most likely will be destroy later in game but it great to help with the high gas and min costs of massing Battle cruisers. Tech you BC to max out weapons and Armor. you only will need 2 starports and get Yamato Cannons, Its a must to do this right.
The Attack. once there is only one Comp left and you have 12+ BC send you your attack. when you see a Void ray use a Yamato cannon on it.
Notes: dont let you battle cruisers attack early due to letting comp know you are there.
Its an easy Achieve if you do it this way. I did it this way and I help my little Brother do it this way.