To destroy enemy wall segments with your troops you can either position your Rally Troop near the enemy wall where your troops will then follow. Having them idle there will cause them to automatically attack the enemy walls. Alternatively, you can press and your troops will move to the enemy walls and begin to attack them.
You can unlock this during a quick match and have a second controller connected to make this achievement extremely easy.
Completely destroy 50 of the enemies walls with just your troops and the achievement is yours.