NOTE: You CAN use instagib (second video) and this can be done on any campaign mission with a loader, not just RAAM's Shadow.
Start Act IV: Chapter 5 and choose the rotor option (left).
Make your way through to where the rotor is and hop in the load lifter. Grab the rotor, go through the next two doors and you'll have to fight a group of locusts. Kill the three little guys (wretch, grenadier, and theron) and reload the checkpoint. Repeat a few times until it unlocks.
Shouldn't take more than 4-5 attempts to unlock. Couple minutes total. Just don't mess with the Mauler, he'll destroy the loader, even on casual.
NOTE: I know the video starts early, it's to demonstrate that instagib is on.