You'll find this on one of the building SW of the Courthouse. It will be marked on your map. This is mostly just a combat mission. When you start fighting, keep the common enemies at bay but focus on attacking the glowing guy (who will remain nameless so as not to spoil). The common enemies will spawn endlessly until you take the main guy out. He will also shoot a lightning attack at you if you don't hit him as soon as he appears. He will then disappear once hit and then reappear again so keep your eyes peeled for him.
Once you've hit him a few times, do a ground takedown on him to finish the Mission
