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.