This is the hardest achievement you wil run into in the DLC. You have to kill every mascot in the activity by going building to building.
You don't have to worry about points and the time only matters when on the actual roof
If you don't have enough time on a rooftop from the mascots running, you can get in a cannon to be shot off and return for more time
Since you don't have to rely on points, you can use the Giant panda balloons on the bottom of the map to launch you high enough to get on buildings again