In this stage you'll be walking around in a giant robot, and you'll need to destroy the large flying autogyros to get this achievement. You do not need to worry about the smaller ones. They move quickly, and it can be difficult to hit them with your shots, so it's best to tackle this once you've played the stage a few times so you know where to expect them.
You can punch or shoot the autogyros with your robot. Each Skylander element causes the robot to shoot a different type of ammo, so find one you're comfortable with. The tech element has a quick firing shot, but does less damage. The life element fires watermelons which do a lot of damage, but are slow.
There are 12 total, four in the first section, four in the second, and eight in the last. After you complete a robot section, your progress to that point is saved. If you miss an autogyro, you can reload the checkpoint to try again.
Here's a video by TikayBeatz PSN, which shows each robot section, and has a counter showing the running total: