You can find six Control Towers. These places send certain signals that aren't harmful to humans. However, they make machines repair instantly. This means that each signal recovers all health points to all machines that are in its reach. Because of that you must destroy the towers.
The best way to do that is to be stealthy. Thanks to that you won't have to fight with nearby machines. You must aim at the object that resembles a core, it hides when a tower sends a signal.
Attacking other parts of those towers is only a waste of arrows. These constructions are completely invulnerable to attacks so you won't damage them in any other way.