This achievement requires that you win a total of 50 battles with only a single soldier remaining. This can be boss battles, random battles, and battles against friends. It does not matter how many soldiers you or the opponent starts with, but you must WIN the battle with ONLY ONE soldier remaining. These types of outcomes are generally rare, but you can easily boost this in the first boss battle against the Barbarian General at any point. From the World Map (Tasks>Click on the globe), click on the Southland Basin and tap the picture of the General. Select Battle #1 and begin (requires two energy). Place only a single, strong soldier to battle the two axe men. Any mid-to-high level cavalry or spearman unit will do. Win the battle and repeat until you reach 50. It only uses two energy each time so this will not take too long. You can check your progress towards this achievement under Tools>Achievements (there is a counter).