You will probably unlock this achievement oganically while playing the game. If a wounded enemey dies when you shoot him in the leg, that counts, so they don't have to be one-shot kills. Even so, you can get one-shot kills to the legs, if that's how you want to do it, with either the pump-action shotgun Lara receives and automatically equips during a fight in the Geothermal Valley or the Voidhammer shotgun. Both of them will even kill enemies carrying shields, with one shot to their shins, without any upgrades.