Isaac creates a small enough projection platform for him and Kaiser to fit, and boosts them onto the roof, granted he has an opportunity not to be seen.
Even with the parasites interfering with magic it doesn't take much effort for Isaac to make a large enough projection for Kaiser and him to ride on. Waiting for an opportunity to not be spotted heading to the roof takes a little longer, but there's enough cover provided by some nearby trees to do so. Once they're on the roof the pair sees a small forest of ductwork and a couple of doors leading down into the building. More importantly, it has several skylights into the interior of the building, including the gymnasium where Dalia is held.
From above Kaiser and Isaac get a good look at the ritual being conducted. A massive magic circle lies in the centre of the room, to hold the creature being summoned. At equidistant points along the circle smaller magic circles have been chalked, each of which has a person lying in its centre. The people are all wearing white hospital gowns, with strange rules marked on their clothing which are separated by a long, bloody cut along the stomach. Dalia stands out from everyone else since she is also hooked up to a respirator and some other medical equipment for an unknown reason. Several people line the walls of the gymnasium, including a couple of security guards, but it's obvious they're just observers. Standing at the edge of the master circle is a man, obviously Michelson, with his arms raised and presumably chanting the words of the ritual. In his hands is a oddly curved dagger, dripping with blood. Whatever he's doing is obviously working, for the centre of the master circle is starting to visibly warp as the Parasite Source is summoned from wherever it was hiding. No one seems to have noticed Isaac and Kaiser up on the roof, but they don't seem to have much time left.