So there is actually a fairly easy way to kill RRAM on insane that myself and a friend figured out a while back that works really really well.
One of you needs to get the sniper rifle (its around the corner in the train before you leave it and get to the cut scene) this player should be the better shot out of the two of you. The other should get the nades/ammo and keep the lancer.
Basically player 2 shoots at RAAM and tosses grenades to get the Kryll off of RAAM. When that works player 1 shoots him in the head, actives and repeat.
Kills him pretty fast in my experiences. I don't have the achievement for it now because it was on my GT before I got live, so I had to make a new one.
