You can unlock this achievement near the beginning of Act 3 during a level called 'Pompeii Pyromania'. There is a part where you need to ascend straight upwards and there are clocks along the walls that you can shoot to slow down time. There will be a seemingly endless supply of 'Gnaar' falling from the ceiling and you can shoot them as they fall to create a platform of dead bodies to stand on. There will be far more than 50 here, so you can stay at the bottom and just keep killing them as they fall or you can ascend upwards and try to get 50 before you reach the top. The achievement will unlock upon killing the 50th Gnaar.