To obtain this achievement you need to win a game using an Defensive legend. Defensive legends are denoted with a shield logo. Currently, the defensive legends in the game are Gibraltar, Caustic and Wattson (the latter two need to be unlocked with either Legend tokens or Apex coins.)
My best advice for winning games would be to find a relatively quiet spot to land with your team and loot up before fighting. Besides that, teamwork is essential. Stick with your squadmates and support them when needed.