when you get to the very top with the psycho throwing things at you go to the left and up the first two sets of stairs, the edge of a wall will be blown away letting you jump off, look down and you can walk off down to it, careful though.
not really good at explaining things so if someone else wants to pick up on this feel free
When you finally make it outside on the roof, go right, up two short flights of stairs and then drop off the roof on the left where the wall is broken. There will be a giant golem or whatever throwing huge firebomb things down at you that explode on the landing at the top of both stairways (including where you drop down from) so be careful. Dashing through is the best way but can be a little tricky your first time since you dont know exactly where to go.
I was so relieved when I first found it, and got killed by a bomb like right there at the ledge because I hesitated to look down for my landing. frustrating.