If the Partisans are standing calmly, walking around and talking, you’ve done it. If they’re still behind cover and shooting, you missed some enemies. Once you’re done killing the rest of the enemies, go back to your allies.

You don’t have to kill every enemy on the map, just the ones attacking the Partisans. Search and kill any enemies you can find. ((!!IMPORTANT DETAIL!!)) -> Help the Partisans kill the enemies attacking them. Those are the Partisans you need to help. You’ll inevitably find some allies fighting enemies. Once you’re done reading the note, open your map and find the objective that says: “Investigate Graveyard”. Once you find the Partisan HQ, you will pick up a note telling you where Sofia is hiding. There are no objectives here for unlocking the rifle. I recommend easy, since it doesn’t matter, and it’s less of a hassle.

Start your new campaign on any difficulty you wish. You can’t just select one mission at a time, you have to play through them in order. Here is a little guide on how to unlock it.

Some people have figured out how to unlock it, while others have not. There has been a lot of confusion regarding this topic.