Assassin is a shot type used to describe back of the head shots as assassins shoot the back of the head for quick deaths and stealth. Normal Assassin shots deal 1.4 damage and can be increased with the Precision Laser. Weapons with boosted headshot damage also have boosted damage for Assassin. For some reason, hitting one above the head counts as "Assassin!" too.
It can only be triggered when you kill someone in the head before they notice you. Or, you can jump on top of them, and firing your particle cannon, can deal them a fatal blow with the fireball rain.