Bringing a gun to bear.

[Kromm]

"Attack by shooting along my designated field of fire if anything moves there" is an entirely valid Wait condition, and so are "Attack anybody who pops up on that roof" and "Attack anybody who walks into sight carrying a gun." The rules are clear on this, what with "a foe moves into range" being the first example, and the third example being expressly that of covering an area with a ranged weapon. As long as you specify the maneuver you'll take and what will trigger it, you're good. Triggers can be as general as you want! If they're too general, of course, this tactic can backfire; see Target Discrimination (p. B390).