For every outmatched achievement, I did the exact same thing:
Only do the Insane matches (The VHard Hard and Medium Achievements come along with the Insane one)
Play on Megatron, it offers a great base that you can hit every enemy from one central position, which leads me to..
Play Protos v Terran. Do a standard Cannon Rush:
Rush one of your initial probes to the center of their 4 base area.
Build a few probes while the rusher is in transit. As soon as he his his location you should have enough minerals to build your first pylon.
The key is to build it far enough from all bases to not bee seen when placing. It cannot be seen when its initially placed, or they will send SCV's at you. If they see it while its building, thats no big deal. They just can't see the initial warp.
Wait for the Pylon to complete, then throw down a forge. Wait for the forge, then start placing Photon Cannons at the perimeter of your lone pylon's power range.
Rinse and repeat. Just keep placing cannons and pylons to expand as needed. Eventually they will quit as you've damaged their game too much.
Warnings
If you don't damage them enough with the initial few cannons, they will ramp into Mech and you will lose.
If they see you place the forge or the pylon, they will mass attack with SCV's and you lose.
Follow those steps and you will be on your way to Outmatched Crusher
For the 'Kin' achievements i had a hard time until i ended up using silly tactics for each of the 3 races to allow my team the win with a lot more ease, and a lot less frustration at my idiotic teammates.
Protos
Cannon Rush. You can see my description of it here.
The basic gist is: send a starting probe to their base (on Megatron) and set up a pylon and forge, then start planting cannons until they are all hampered enough that your team rolls over them.
Zerg
Spine Crawler Rush.
This works best on maps where all the bases are seperated.
This means your opponents all need to be zerg.
Harvest for < 1min with all of your drones. Build 1 or 2 new drones. Then save up for a pool. After the min has past send 2-4 of your drones into the opponents base. Then split them up 2 per opponent base. As soon as you have the resources turn those drones into spine crawlers behind their resource line. Use the minerals to funnel the drones so only one can attack your spine at a time.
If all goes well you will land 2 crawlers behind 2 different bases and it will cause enough economic damage that the other 3 players on your team will be able to roll over the opposing AI. If all does not go well, they will have attacked your crawlers and killed them. Save yourself the time and pack up and surrender. Just try it again.
Terran
I chose Protoss for my opponents on this one.
This works best on maps where there are two players per base.
I built fast into 3 barracks with tech labs and mass produced Reapers.
I then proceed to reaper harass the opponents, watching for striders and zealots to come defend while cleaning house of probes. If you are able to take out a nexus early, that's always very helpful.
If all goes well you did enough damage that your buddies can clean house, otherwise start over.
Hope this helps out, this is exactly how I did them.
Best Answer
Blizzard recently added a new custom map which makes it pretty easy to do this using the map Burning Tide.
A full guide to beating the insane AI on this map can be found on YouTube (also read the description).
This method worked for me in patch 1.1.1 - I'm not sure if it's still working but you should give it a try ;)