"Wall Miners are specially trained to attack enemy fortifications. Skills included digging trenches to safely advance artillery and infantry positions. Wall Miners also dig mines under enemy fortifications, causing the walls of the structures to collapse, allowing the attackers to get inside and kill the surprised defenders. Historically, the defenders would dig tunnels to intercept the attacking sappers before they collapsed the wall. If two tunnels met, the underground fighting would be bitter and without mercy."
Try to use two Veteran Wall Sappers to destroy one wall. That way, if one Veteran Wall Sapper dies before it can destroy a wall, the second wall Veteran Wall Sapper can destroy the wall since it used the first Veteran Wall Sapper as a distraction.
If a base has multiple compartments, it is suggested to bring many Veteran Wall Sappers to break them.