The Koopa Troopa is one of the more well-known enemies in the Mario franchise, although it is a bit on the scarce side in 3D World. Most of the time you encounter one of these, they will start chasing after you whenever you get close enough. In very few instances, they walk a set pattern. Whenever you attack a Koopa Troopa, their body and shell will separate and they will quickly try to retrieve their shell. If you get rid of their shell while they’re not in it, they will revert back to chasing your character.
While out of their shells, you can attack the Koopa Troopa again to defeat them for good. You can pick up their shells and throw them, where they will bounce off of walls and defeat any enemies with which they come into contact. Also, you can enter the shells, which is done by holding them and pressing either L or R. Whenever you hop into a shell, you will automatically begin moving, as if you had thrown it. If you stay in the shell too long, you will get dizzy and automatically exit from it.