In the bat cave, right when you start (in the forensic section), there are three switches on the left. Flip all three to turn on the batarang trial. You'll get a mix of target types. Avoid hitting the red ones. You get time bonuses and points for hitting the right ones, lose a point for hitting the wrong one. If you're aiming in a direction without red targets, you can flick pretty quickly from the waist to save time, if you're trying to avoid a red one it's probably easier to pull back and aim slightly. Throw toward the outside of the good target, away from the red targets when you can.
It's pretty easy to get a score of 30 once you get a hang of it.