You'll need to go to a building north of the location of the Radioactive Subject and climb to the top using stairs inside the building and then some parkour in the alley on the north side of that building. There may be other ways to reach the top, but I had to jump from the balcony, then climb a pointed chimney (jump and glide). Once you get on top of the building and head to the south face, you can reach the platform where the Radioactive Subject sleeps by jumping and gliding, followed by a quick 2-3 Hovers forward. You can get a third charge by using Time Spiral.
There are a few gear spokes that stick out at the top that can be used to get line of sight on the hunter to bring them up with Rescue. I've included screenshots of most of the steps: hope this helps somebody! Enjoy your awesome rat.
