Make the most of Lynch's condition.

In the level where you are in the streets, Lynch will start halucinating and all of the pedestrians will turn into cops. Kill all of the pedestrian/cops as Lynch and the achievement will unlock. I hear that it is also capable to do it in the bank when Lynch freaks out the first time. *FUN FACT* When playing co-op and Lynch freaks out, go over as Kane and the pedestrians will be holding imaginary guns.