To repair your wall segments, you need to have your own troops repair them. To do this you can either position your Rally Troop near your own wall where your troops will then follow. Have them idle and they will automatically start repairing your walls. Alternatively, you can press and your troops will return to your walls and begin to repair them.
You can unlock this during a quick match and have a second controller connected to make this achievement extremely easy.
Repair 50 of your wall segments back to 100% health and the achievement is yours.