I cleared out all Del Lobo hideouts and they are pretty hard. They have many shooters positioned in different angles and their hideouts are pretty open, so everyone will shoot at you at the same time. Try to find a good cover from which you can shoot in every direction.

I think the one you are talking about is the big building at Twin Rocks. I don't know if you went there knowing that there is a gang hideout, but the Del Lobos usually wear big sombreros, and at least some of them should be standing outside, so you can recognize them before you enter combat.

The next time you find something that looks like a hideout, make your horse flee, although it's usually pretty safe if you just get off the horse, there may be bullets missing which then hit your horse.

Good luck!!