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 the .
If a fireball hits a segment of any castle wall, friend or foe, it will damage it. After taking enough damage, wall segments will be destroyed. This will likely come through normal game progression, but you can attempt to affect the direction of the fireballs with the way you deflect it if you wish. You can also grab a fireball when it hits your shield by pressing allowing you to grab it and charge it up for a power shot. You can move your shield while holding the fireball so that you can aim it where you please. This also does a lot more damage than the regular fireball so you only need to do this a few times to destroy a wall segment.
A good mission to boost this is the very first mission of the single player campaign. There are 4 players and the only goal is to destroy the enemies castles and the only means to do this is with the fireballs that the Dragon drops. I personally destroyed 17 wall segments in a single match, so boosting this here, although ultimately unnecessary due to the ease of unlocking this achievement through normal game progression, will make this very quick and easy.
You can unlock this during a quick match and have a second controller connected to make this achievement extremely easy.
Destroy 50 enemy wall segments with fireballs and the achievement is yours.