This achievement needs you to win 15 battles without any member of your expedition loosing all his endurance and becoming incapacitated (reviving him won't count and you will still achieve a simple victory).
While it's true this have to be 15 battles in a row, whithout losing one or achieving a simple victory, they don't have to be different battles. The easiest way to achieve this is to play the Hispaniola campaign up to the Tournament quest (you have to be polite to Noriega during the barricade training quest "Tinkering" for it to unlock), save before starting the tournament and then do the first two battles of the tournament again and again by reloading your previous save. You could do the entire tournament, but the third battle is way more complicated than the two first (which, if you use three veterans, are some of the easiest battles in the game).