You have your walls destroyed by the enemy when they attack with their troops or when you fail to deflect a fireball and it hits your wall. Eventually, after a segment of wall has taken enough damage it will be destroyed. After all of your wall segments have been destroyed, your warlord will be exposed and if he takes a hit with a fireball, the game will be over and you will have lost the match.
You can do this in a quick match and have a second controller connected to make it somewhat more manageable. Use this setup;
Mode: 1 on 1
Stage: Random (This is not important)
I personally did this against the easy A.I., but it would be even easier to have a second controller connected. When the match begins use your main controller to grab the fireball with and keep hold of it until your wall is destroyed. After you have grabbed the fireball, use the second controller and press to send the enemies troops to attack your wall. Then sit patiently until the enemy troops have completely destroyed your entire castle walls, at which point have them move away from your castle and still using the second controller, move the shield away to the side of the castle. Then with your main controller, release the fire ball directly at the center of the enemy castle and it should bounce back in a straight line back to your shield. Grab it again and repeat until you have destroyed a piece of the enemies wall, at which point send the fireball into the gap you have created in the enemy wall and it will hit the enemy warlord and win you the match.
The achievement will unlock upon winning the match.