The Magnetar is a Survivor weapon which launches electromagnetic, gravity-nullifying projectiles; these projectiles are able to bounce off nearby surfaces into enemies. It is still in the experimental stage, and was invented by Nikola Tesla.
If shot at the Watchers, it will cause their robotic suits to malfunction, effectively paralyzing them. Use the time you have from their paralyzation to take them down with another weapon.
If you wait long enough, however, any Watcher eletrocuted by a projectile from the Magnetar will eventually explode (except for Watcher Knights, Archons, and Watcher Flying Discs) due to the strong electromagnetic field its projectiles create. This is the easiest way to kill Watchers using this gun, since you only have to shoot them once and never have to worry about them again.