THIRD ASSIGNMENT: Secure the roof
Map legend: 1 - starting area; 2 - enemy sniper and your destination; 3 - ammo stash; 4 - third kilroy
Completing this particular mission objective is only going to take you about one minute. You start the game to the east (1 on the map) of your target. Choose a machine gun from your inventory and start moving forward. You must eliminate one sniper (2 on the map) [#139]. DON'T examine his body right away, because you'll be able to perform two other activities in the meantime.
Travel north-west and you'll come across a new ammo stash (3 on the map). Obviously you should collect much needed supplies. Move to the north and you'll find a third kilroy (4 on the map) on the eastern wall [#140]. Now you may go back to the body and you'll be rewarded with a new cut-scene.