Create a game of team sharpshooter or just normal sharpshooter, the map is The Streets.
There must be at least 8 people in the game for this to work! Tell everyone in the game to equip motion sensors!
As if you are in the alpha base, go up the top left building where the stairs are and the balcony with the rappel.
Go past that and into the small space near the bravo base where there is a box, tell everyone in the game to meet there, then one at a time everyone put all 3 of their motion sensors on the front of the box.
If done correctly there should be 24 motion sensors in the 1 place, then if you walk into the motion sensor you will be on top of the box, now you can walk around the roofs of the streets map!