Just have someone be a decoy while the other person moves for the Troika on the far opposite side of you. Let him unleash the Kryll on you and just hold down the trigger on the Troika. He'll fall into chunks faster than the Kryll can fly to you.
I still occasionally jump into random Insane Cooperative games to help people for just this reason, as AI Dom is too much of a liability, and not many people play that game anymore.
But the hardest part of Gears of War is driving the junker on Insane...especially in Cooperative. If there's one thing Gears of War 3 did correctly, it's not having a driving sequence.
I could never get the troika option to work although I only tried a few times and it seemed impossible so I just did it the old fashioned way.
Oddly enough I only died once in the junker mission on casual when I couldn't tell where the kryll were coming from. Not once on the harder difficulties. After hearing all the horror stories about it I was ready to crush a controller in rage, but musta gotten lucky.
There is also the RAAM glitch where he gets stuck by the block, i think thats how i beat it on insane. I really cant remember. Me and a friend beat RAAM recently and it was easier than i thought. Just load up on torque bows and nades!
Don't forget the longshot as well.