There are far too many solutions for this achievement, and they all make it harder than it needs to be. The last chapter in the mansion, the darkness takes over control
quite a lot. You are outside and the darkness takes over once again and pulls a helicopter out of the sky, this is pretty memorable!
- After a few flashback sequences you materialise in a hallway with a guy in front of you. Pop him in the head, and look up and shoot the light out above you.
- Select your black hole darkness power (blue), allowing this to recharge fully. It stops once the snake heads stop drawing in power / smoke.
- Pop your head around the corner and but dont move forward, there is a couch / sofa at the end of the short hall where there is an infinite spawn.(if you are on hard, keep in cover)
- Wait for 2-3 seconds until and you should see two or three guys take cover behind the sofa and walls, this also allows time for several others to spawn, then throw in a black hole aiming behind the sofa.
-....wait for it...Bam! Gunslinger & Bringing People achievements pop together.
If you need a visual reference point, Foofighter18's video on 9m 05sec is where you should find where I mean. Just DONT run down the hall like he did...!
EDIT for Hexa's valid comment: Don't move down the hall as there is a save point and this will 'screw up' any chance of a repeat attempt if you fail. Just restart check point if it doesnt pop first time!
You Tube Search: The Darkness: Chapter 5 (Part 5 of 6)
Hope this helps!