The only unit that deals enough damage to kill a wall in but a few seconds would be the vanguard. Infernos also work; your opponent could have cut some corners and used them in conjunction with combat fabbers. Fabbers themselves can destroy any wall, though. The commander can reclaim an enemy wall at an incredible rate, and a combat fabber can do it even faster. Leaving just one in your army can mine any wall, no matter how thick or how many combat fabbers are set to repair it.
