I'm trying to do some of the challenges here and one of them involves shooting the dealer and king dude in one shot. I am trying to get to dealer's apartment and instead of there being 1 cop, there's 3. I usually go to the left to get that cop uniform, IF, I can get behind the other cop. There's about double to triple the amount of cops of any video I see.
BTW, I'm playing on Purity difficulty. The vid I looked up said it was purity difficulty as well.
If you run through the double doors at the start, sometimes you'll get the cop there to say something to you as you run by. This makes him take a little longer to get to the spot where the other cop is, so you're free to snag a disguise more easily.
If he doesn't take longer to get there, he will turn toward the building when he turns around to take guard position so you can walk between his back and the boxes without him noticing and walk up behind the other cop.
if you go near the area with the King's car, eventually the 2nd guard will walk up the stairs. Once he starts moving, throw a bottle to make the other guard move. Now you can take out the guard at the top of the stairs, take his outfit, and blend in with the guards to get to the drug dealer's apartment.
Otherwise, normal difficulty doesn't have those extra guards and you can still get the challenges.