First of all, you will be needing a spear with the slide attack (square, square, circle) and a Iytei I.D. with shoulder ride. Once the boss in the infinite corridor (Dullahan) is defeated; the area in the Forest of Jigramunt with the western toilet will be available.
Use "shoulder ride" and aim towards the space between the toilet and the slanted wall. After going into that spot, steer back and forth until the Iytei's feet sink into the ground. Then jump off the Iytei and perform the slide attack with any spear--but make sure you are holding the joystick in a direction that will make Hector slide up the slanted wall.
Try to get as high up on the wall as you can, and press triangle rapidly to engage the shoulder ride ability again. If timed correctly Hector will appear on the ID's shoulder's for a brief moment and then jump off. But Hector will now be on the other side of the slanted wall--under the walkway that leads to the save room.
Trevor can also do it as well!
Stand next to the western toilet chair in the Forest of Jigramunt, face to the left and execute the "square, circle, square" attack. If done properly, Trevor should do a cork-screw attack--and if you are perfectly lined up with the wall, he'll go right through it!!
Of course there is no percentage gain and Trevor can't use teleporting items like Hector, so you'll have to go under the slanted walk-way (at the end of the video) to get back inside the normal game area.
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