Before you enter this impenetrable fortress, there's a couple things you should probably know about how this works. Essentially, you are barricading yourself in the enemy frontlines, and since you are closest to the enemy, they are programmed to throw grenades at you and only you. As long as there are no distant enemies in front of you, grenades will not kill you. Additionally, any enemies in front of you will be shot at by your patrolmen, oftentimes preventing them entirely from gaining the opportunity to throw anything. Since most of the enemies are focused on you, you do not have to worry much about your allies being shot, except for ones that ignore you; however, those that ignore you are exposed entirely to you if the outhouse door is open, and they will have no cover from you; if you are worried your allies will fall during an ambush anyways, opening the door to take care of any enemies that go for your allies will suffice.

The first thing you need to do before entering this outhouse is to take care of the gas canister outside. For whatever reason, these explosives can damage through walls, or at least in this instance. Destroying it beforehand ensures you won't be blown up.

As you may have noticed, there is a heart-shaped hole that is conveniently at eye-level in the outhouse door. When closed, it not only provides you cover and still opportunity to headshot any enemies, but it causes enemies to cluster around the door, giving your firing squad a huge target. Don't let them take care of all of them, though, because eventually there could be too much. The sniper ally helps a lot for this, but this method still works with default upgrades. Just center your crosshair on the heart and shoot whenever an enemy walks in front of it.

Oftentimes, enemies may open the door, but for whatever reason, half the time they don't even shoot at you anyways. Closing the door can be difficult sometimes with the mound of corpses that gathers right in front of it, but staying inside is still the best option for this scenario; just wait for the bodies to despawn and close it again.

That's pretty much all there is to this guide; if you have any questions or issues regarding this, let me know and I'll try to help. Do also understand that no solution or method can be perfect, and that there's typically exceptions or circumstances that serve as weaknesses for them; while the preparation section eliminates the largest exception to this increased protection, it mitigates risks as much as possible, not completely removing risks. I wouldn't be surprised if there was another way to cheese these ambushes, but currently I enjoy this one too much to look for any others.

Also, here's a few screenshots showing just how ridiculously broken this method is: