During matches a Dragon will swoop down and let fireballs loose on the battlefield. You can deflect this away from your castle with a shield that you have control over with .
You need to grab a fireball when it hits your shield by pressing allowing you to grab it and charge it up for a power shot, to kill an enemy troop. You can move your shield while holding the fireball so that you can aim it where you please. This will allow you to aim at the enemy troops directly. Normal fireballs will not hurt the enemy troops, so you need to kill 50 of them with a charged shot fireball.
You can unlock this during a quick match and have a second controller connected to make this achievement extremely easy.
Cause the death of 50 enemy troops with fireballs and the achievement is yours.