This can done easily on Chapter 3, near the end of the level, in the turkish baths. Follow the storyline through, after you have picked up the brief case,(there are plenty of guns/ammo on the shelves either side of the brief case) you have to re-trace your steps to leave the area and return to the City hall station. On route, in the two level room with the scaffolding, a chopper hovers over head. Becareful as SWAT guys repel in, deal with these guys from the hall you approach from, there is a darkling spawn here too. When they have gone just launch 2 or 3 BLACK HOLES in the direction of the chopper and it should go down.