You can get this one as soon as the very first mission in the game. If you shoot or kick a vending machine it'll rapidly fire out cans which will stagger enemies. You'll find one after taking an unexpected ride on the heavy vehicle and another when you chase the target up the stairs. As long as an enemy is standing somewhere directly in front of it you can hit to shoot it automatically and stagger them. If you don't get this in the first mission, there are Vending Machines everywhere and it isn't too difficult to lure an enemy near them to get this trophy. Occasionally crimes of opportunity will involve crooks trying to steal from vending machines, if you come across one of these you can just shoot the machine to easily get this trophy.