Instructions
Turn right from the spawn point and approach the ladder by the doorway. Position yourself at the bottom and face the ladder. You need to be facing the ladder exactly, as you cannot make any fine adjustments with the right analog stick as you climb and descend.
Climb about halfway up the ladder, taking care not to touch the right analog stick at all as you do so. Stop at the halfway point and descend back to the ground level. Keep holding back on the left stick as you touch the ground in order to step back from the ladder.
Strafe over to the doorway that was previously locked. Use only the left stick to move, and continue to avoid touching the right stick. Head toward the door and press the triangle button to dive through it. You will pass straight through the door and out of the level.