Where's the best place to farm the Behind Closed Doors challenge?

Question

In Splinter Cell Conviction, you can earn some P.E.C. points by killing 50 enemies whom you've marked while using a mirror or a snake camera from behind a closed door. It is massively annoying to use the snake cam on every door in hopes of finding one or two enemies behind it. How can I get this challenge out of the way as quickly as possible?

Answer 1

During the mission White Box Laboratories, you will use an elevator, after you leave it you will immediately come to a door behind which are 5 enemies - 4 in front of the door and 1 to the left. Crouch and roll to the door (don't run, you'll tip them off), use the snake camera on the door and mark all the 4 nearby mercs (or if you don't have a stored execution, instead of marking the closest merc mark the one on the left). Smash the door and use the portable EMP, then execute them (or use a takedown on the unmarked merk, then execute the rest). Reload checkpoint and do it again until you get the challenge.

You will need an upgraded Five-seveN pistol to mark 4 targets, luckily there is a crate which can be used to equip and upgrade the pistol.