Close to the beginning of the game (Chapter 1-1), you'll come across a sign that reads, "Pueblo." Go down the road, and you'll find a village full of hostile people (they don't notice you at first, however) that you view through binoculars.
What you want to do first is just run past them into the second house on the left that has an open door, which will trigger a cutscene. Once it's over, block as many doors as you can and grab the mounted shotgun on the second floor. Also, any ammo that you can find because you're probably going to need it.
To add on to that, eventually the villagers are going to break through all the doors and windows, so you may want to start killing/evading them on the move. There are various herbs scattered all throughout (at least one of each color, so you can combine them to fully restore your health and raise your maximum health by a small amount). Ammo will of course be found in the same way, various places around the village. There is a box of shotgun shells in the top of the tower which you can climb, but I wouldn't recommend staying up there because explosives will start to be tossed in the top room and they're not very easily avoidable, if at all. It's a long climb, but don't worry about taking any fall damage.
All you have to do is hold them off, and then Dr. Salvador (aka the chainsaw guy) will show up. He's very easily taken down with a hand grenade and a few shotgun blasts. Continue killing off the villagers until another cutscene triggers where the church bell rings, after which all of the villagers will go inside and the achievement will pop.
*Alternatively, if you stay outside, Dr. Salvador will still spawn and attack you. If he does and you go inside, the other will spawn, though it's doubtful two will be attacking you. Just if you would kill him and then want to do it again. If you don't kill him, he will be replaced, but it's unknown if he'll have the same health as he did before you went into the house.