Stapler Frenzy is one of 28 Achievements in The Respawnables.
The description says Kill 25 enemies with the Rookie Machine Gun
This achievement is earned after getting 25 kills with rookie Submachine gun. Note that only enemies in the Mission Mode count as enemies.
Tips & Tricks
This gun's strengths lie mostly in its agility, so if you must use this weapon, use the Run And Gun technique and evade the bullets as they fly towards you. In close combat, try and constantly loop around the opponent to confuse him, throw off his aim and extend the amount of time you have to deal damage to him.
A detailed strategy about the Rookie machine gun can be found here.