After about the first twenty minutes you will reach what resembles a destroyed parking garage. Note its location. Complete the mission, then return there. Shoot the airship to get its attention. After it shines its spotlight on you, wait. Note: A few KPA may get in, however if you sit at the top of the first ramp you can walk up you will have enough time to take care of them.
At the start of the "Zero Hour" mission is a small area that you will clear before being told to retreat. Hide behind a car instead and wait. Only about four KPA will appear in Deathwish mode.