The first of these side missions came for me after the Arkham City Sirens achievement and the last one came after The Wrecking Ball achievement.
While you are running around the city, Z will call a phone booth nearby. Use detective vision to locate the phone, then answer it. Z will give you a another phone booth to reach in a time limit. I recommend using the map to find the goal, then run to it. It'll be another phone booth, so it will on the ground.
I had plenty of time on many of these, and got all of them on my first time. It's not hard. After each time you do this, there's a minigame. After 5 or 6 times, you'll find his lair. In his lair, there's a lot of jumps that aren't too hard. For the final jump, you need to use the line launcher, then drop and quickly grapple away. The timing is tight on this.