...look up from where you start off. Use the sniper rifle to shoot the very high up red-glowing flying enemy. Then you'll auto overdive on the ground. Once that's done, the game will put you back in your original position on the rooftop. Next, overdive into the officer across from you on top of the building (overdiving is the only way to get to the roof of this building, so protect that guy as much as you can). You'll be able to shoot the next 2 red glowing enemies from that position. I think there was one more red glowing flying twisted that you'll have to hit from your original position. As you take then out the poison gas will finally dissipate. When in doubt just look up. The twisted you need to take out are flying higher than the other flying enemies.