How do Mark and Alec gain access to the interior of the Berg in Chapter 5?

Mark is given a gun by Alec, with orders to cover him. The Berg, hovering over the city, draws Mark’s fire, causing three of the attackers to seek cover. Mark carefully aims and hits one of the attackers in the head, and his body tumbles from the Berg. Alec, meanwhile, fires a grappling hook to the Berg, takes the gun from Mark, and is propelled up to the Berg.