You now have to return to the church. The place will be well-guarded but fortunately, you’ll be able to infiltrate it through one of its many entry points. You can sneak through the church’s side doors, the hole in the wall in front of the main street, or the broken part of the wall behind the houses. Like before, it will make it easier for you to whittle down enemy numbers through stealth before deciding to go loud and engage enemies in a gunfight.
So far, a good entrance is through the broken wall to the southeast. This gives you access to the garage that has a weapon locker with Tac-Vest, a medicine cabinet, and lets you get within range of a sentry. On the rooftop of that garage, you can find a Manitou Model 67 which you can use on that spot or on the sniping point on the rooftop of the building where the heroin is stored. (Use Intel view) to find this sniping point. Take note that once you’re detected, the enemies will attempt to flank you by climbing to your sniping area or throwing molotovs at you. This is actually easier since you can ambush them on the way up; just use your intel view more often.
Clear the guards outside then head to the heroin warehouse.Take note that there are at least three gang members inside the church so they’ll come looking after you eliminate all enemies outside. There's a medicine box in the small room beside the entrance and you have to break the door's lock. There’s only one guard there and he’s also distracted so you can easily sneak behind him and kill him or simply whack him from your position.
Some of his men will arrive as reinforcements but these are only a handful that you can easily deal with. When ready, interact with all four crates of heroin to destroy them. Don’t forget to grab the Vargas painting in the small corner office with some cash. Mission complete.