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Simple lose on the first mission, just go find the first soldier and stand still until he kills both of your soldiers.

BUT!

This is best doing at the very beginning of your campaign or after you finish all 3 chapters...

The reason for this is as you play the game you soldiers level up making them stronger, unfortunately if they die you lose them completely and the game gives you soldiers with lower rank. So if you haven't obtained the achievement from start, it would be a shame to waste your leveling up and is best to get it once you are done with everything else.

Good luck :)