This achievement can be obtained on ANY difficulty and requires you to complete the fifth mission on Act 1. the mission in question is "Takedown". At the start you will need to chase down a suspect and shoot him in the legs to imobolize him. I would advise you not to shoot any of the civilians otherwise its game over. For the next session you will be going alone pretty much so you have to navigate a lot of close areas with lots of corners. Slowly pick off all the enemies on your way (don't worry they won't keep respawning). No matter how slow you go Rojas will not escape and at the end you will see a sweet piece of slow mo.