Enter through the upper door, walk down the corridor on the right, go down left into the room with the billiard table and mafia wizard, grab the ammo and the damage up barrel from behind the billiard table, wait for him to finish his burst and then run back to almost the start, oneshot the muscle man guy at the entrance and take the silencer from his stash, go through the upper right corridor again and kill the guard that's down there with the silencer (otherwise the muscle guy in the corridor leading to the upper target will turn towards where you'll be coming from, making everything much more difficult), grab the ammo he's protecting and go straight down, shoot the muscle guy on the right, get the unguarded ammo from the room above the current corridor and go all the way to the right until you reach a duplicator barrel. Equip it and now you can take out the muscle man target above you (we never really came up with a plan for this one, just trying to stay as far away from them as possible with the duplicator giving you a chance to kill them after missing) by taking the left path so you only have to kill one and not two of them before reaching the target.
The order in which you take out the targets after getting the duplicator doesn't really matter so I'm just going to list them:
1. Mafia wizards
Stick to the wall and take out the upper guy as quickly as possible (with the duplicator and dmg up barrel a single bullet will kill, provided you hit with both projectiles). Then target the lower one, if you're fast he's not done turning around yet so finishing him off just takes another clean shot from the back. The target will have probably noticed you by now, so wait for his burst to end and go in for the kill, you can keep distance and take advantage of his short range to find an opening. The second duplicator is neat but ultimately pointless.
2. Regular guards
Unequip everything but the silencer and take out the upper guard while he's still turning towards you, for the other two just don't get hit, duh (sometimes when using the silencer the third guard won't notice you until you're right behind him). Grab the ammo the third guy was guarding and reequip your dmg up barrel.
3. Serpentine corridor
Take out the muscle man and grab his ammo, then unequip the dmg up barrel for better maneuverability and take out the two guards in the corridor. Go back to get your damage up barrel and pray to god the business man guy with the dual pistols won't be able to turn around fast enough to catch you trying to oneshot him.
4. Watch the game freeze.
I hope I remembered all of this correctly, I spent like an hour (or more, time flies by) with a friend trying to crack that last level and it was super rewarding when we finally managed to pull it of. Good job guys, was a lot of fun. (also I hope there's only four levels because I lost count so I'm just going to assume the 4rth one is the last one, otherwise this comment was a massive waste of time)
Also, did you use a custom engine?