You can do this on any mission except the first which you can't choose gear before deployment. simply equip each squad with only ranged weapons, note that lacking a weapon in a melee slot doesn't mean the squad doesn't have one, so equip two handed guns on those squads. During the mission you can choose to melee with your ranged weapons without it effecting the outcome. Simply beat the mission and this achievement will be yours independent if you go into close combat or not.