For example, if your sharpshooter's pistol has a damage range of 3-4, and your choices are one target with 4 HP and 100% to hit, and another with 3 HP and 95% to hit, I would go for the 95%. It's often better to go for a lower hit percentage on a lower-HP target than a 100% hit chance on an enemy that might not score a headshot. You'll want to be aware of your weapon's minimum damage and the target's hit percentage. It helps a lot to start off getting 7 headshots without using any actions or ammo. I used my Templar and my sharpshooter first, because their autopistol and pistol have infinite ammo without reloading. When I began making headshots, all of my soldiers were more or less together, and there were about 20 Lost surrounding them on all sides. Since I was running without making any attacks, some of the Lost got within melee range and attacked my soldiers with their little sissy slaps, which is fine because close range just makes them easier to kill. This map had an Advent train on it, which worked out well as a shooting platform. Once I found them, I didn't shoot any Lost, I just dashed my soldiers towards a center point where they could all get on high ground. What I did was send my soldiers out in two groups of three to find the Lost pods. Once the mission is started, you may want to make another save that you can scum in case you don't get your 15 headshots on the first try. No problem, that's why you're making a save, remember? After you've saved, change your difficulty to Rookie, select and load out your squad, and begin the mission. (This means the map has only Lost no Advent or aliens that you have to fight simultaneously.) You may see this sitrep on one of your guerilla op choices when you actually wanted to choose a different mission. Next, wait until you have a mission with The Horde sitrep. It's more about having a fullly equipped midgame-level squad you're comfortable with than about having a specific trick up your sleeve.

I don't think you need to worry too much about bringing a certain weapon, mod, squad member, etc. By this time, you will also have a decent amount of stats, abilities, weapons upgrades, etc., all of which will help. Wait until you have a captain and can buy the Squad Size II upgrade. You'll probably want to make a save before you begin, so after you unlock the achievement, you can go back to where you were and finish your campaign the way you wanted to. I don't recommend trying for this achievement on Ironman. The Video highlighting some of the points:

Take some Sharpshooters (only really valid if you have the 'Between the Eyes' card or you pick and choose targets but Sharpshooter pistols don't need reloading so you can shoot as many as you can in range) Take your highest ranking soldiers, the higher they are in rank the greater their aim stat Get height advantage to increase soldier accuracy Equip your squad with as many Auto-loaders, Extended Magazines and Scopes as possible, auto-loaders and extended magazines mean chances for consecutive kills from one character without running out of ammo. Use the 'Between the Eyes' Faction Order available from the Reaper Faction that allows any damage on Lost to be a 1-hit kill. There are a couple of things you can do to make this far easier: Once you've started taking out the lost make sure you're shooting only using the 'Headshot' action that replaces standard firing for this to count towards the achievement. Once you break concealment the lost will move closer too so make sure you account for that. Once you have the mission, place your squad in the best positions possible around the map so you're sure to have at least 15 in sight.

Hopefully you don't have the dark event which means your squad isn't concealed at the start of the mission but even if so its worth bringing a reaper to scout out the mission to find all the horde locations. Having tried several approaches I'd say the easiest guaranteed way to get this is to wait for a mission with a sit rep of - 'The Horde' - which means the mission is lost only. This isn't quite as simple as just waiting for a mission with lost and using explosives to trigger more lost.

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