Instructions
Destroy all of the Lego baddies you see in the Scarecrow section. Destroy all the stationary objects you can. You'll not only get points, but when they are all destroyed, you will see purple bombs you can use to defeat Scarecrow.
Pick up the bombs in the lower right side of the screen next to a gun. Catch Scarecrow using your batcopter. As soon as you catch him, switch over to the batwing and use the bombs you just found.
Scarecrow fires multiple rounds from his biplane. The easiest way to get around the shots is to glide to the left around the slow but lethal bullets.
Hit the scarecrow three times with the purple bombs you picked up, and Scarecrow will be defeated.