STEP 1: Walk/Sprint up against the wall/barrier
STEP 2: Crouch and turn 45* to your right and move a tiny bit forward until you cannot stand.
STEP 3: Look up or down and make sure that you aim on your gun is basically (not quite) parallel to the top of your tv or a line on the wall/barrier. and then turn your stick layout to Legacy Southpaw.
STEP 4: Move Left Slowly until you can stand and move back and forth between you standing and cant stand extremely slowly until you hit the elevator.
STEP 5: DONT MOVE WHAT SO EVER. Position your index finger on X or A and your middle finger on O or B and tap the X and O[ps3] ( A or B[xbox]) practically at the same time and tap O or B straight after you just tapped it and you should hit the jump elevator as well.