Sitting like a duck on top of said billboard, kicking his heels...
He's startled nearly off of his perch, and takes just a split second to sign off before catapulting downward like a vengeful rock thrown by an aesthete God.
His fist meets the mugger's face with a satisfying and frustration-venting WHOMP!, and the mook goes down way too easy. He looks around but for once, the scum of Gotham's boots isn't coming on in swarms, and the only other person in the alleyway is. . .