First thing first:
COVER IS YOUR BEST FRIEND IN THIS GAME! Just like any other game, but Gears of War especially, cover severely impacts your performance on Insane difficulty. Always evade, recover your health, do good.
-Take it nice and slow too, no need to rush.
-The snub pistol is not useless!!!
-Chainsaw is good, in emergency situation. You might already know this, but if enemies are firing at you, your chainsaw will not rev up.
-Shoot, cover, shoot, take more cover, shoot some more, repeat!
You can do this on co-op and have another person on a different difficulty, for example Casual. Hmmmmm...nice. Just have your friend do all of the work if you want to earn this this way. You can also hook up two controllers, and play casual on another account.
Beast of luck and cheers!