Use the print screen jump on any building located near the red dot in the image below. Face the wall, and be staring at the building on a flat side(harder to do on corners of walls). Make sure you are not at an angle (or you will travel down the wall while doing the print screen jump). I find it best to print screen jump on what looks like a support strut running up the side of the wall. Spam print screen and jump alternating VERY fast. Do it the right amount and you will end up falling into the building/into the water beneath the world.
From there you just swim around and attack everything. Monsters in CH15 PQ don't evade bug. Easy farming for inf+chest rewards.
You will need to decide when to stop stacking the printscr+jumps to properly fall into the building. I cannot tell you when to stop because it's different for each persons computer(the printscreen jump bug relies on interface lag that causes your jumps to stack onto each other while near an almost verticle surface : It relies on how long it takes your computer to screenshot)
It takes some practice to get print screen jumping working. Exploit does work. For those who are not used to print screen jumping it may take a long time or you may end up on the roof of the building instead. I can reproduce this exploit and will provide screenshots later.