This will probably prove to be the most difficult campaign achievement. You can do this in local or online co-op. You must defeat RAAM without either playing being downed on the Insane difficulty. As the most common strategy goes, one player uses the Torque Bow while the other snipes with perfect active reload head shots. The exploding arrows disperse the Kryll around RAAM leaving him vulnerable to fire; lay into him with head shots.
Optional: your teammate can switch to a primary weapon, like a Lancer, and put some extra damage on him in between Torque Bow shots. As he moves closer, use grenades and pump him full of lead! Should you fail, just reload the last checkpoint and try again.