The Bully skillshot is achieved by knocking an enemy over with a cannonball from the Bouncer and then detonating it, killing the enemy. This may sound difficult, but it's actually pretty easy. When you fire the Bouncer, aim at the enemy's feet, and hold down the fire button to keep the cannonball from exploding. The cannonball should knock the enemy over. As soon as that happens release the trigger and it will explode, killing the enemy.