Insurgency: Sandstorm

This one in quite doable despite what you think! It took me a few tries, but after around 4 hours I got it.
Play Coop as it is the easiest way to get it
Just look out for doors on maps, if you have random teammates just try to stay seperat from the group or your teammates will probably kill the enemy before you can get him with the door! Try to look on the map from where the enemies are pushing and position yourself at a door between the pushing enemies and the objective. The best way is to keep the door open so you can see them pushing in your direction, as soon as they noticed you and are close enough just shut the door and wait till one of them is on the oter side, ususally you can hear them speak and most of the time they are announcing themself opening the door, right at this moment you simply kick it open and kill them with it!
Be aware:
- You can't kick doors when you are out of bounds (like you can't fire your guns)
- The enemies can kick the door right in your face
- The enemies can shoot through the door

Hope this helps!

For those trying to get this by themselves, the Hideout map seems to have most of the doors in the game. Objective C for Checkpoint Security in particular spawned an enemy behind the door next to the counter twice in a row for me, but I haven't tested it enough to say if that's guaranteed or not. If you're having trouble getting to places before your teammates, you can drop your weight to get a bonus to speed. Just be careful that you don't become so vulnerable that you die before even finding a door. The SMGs and PDWs that Breachers and Advisors have are good for lightweight, close-range firepower, but you can also go Rifleman and do pistol-only if they're filled up (I'd go with the PF940 with extended mags for the extra ammo).