It can also be upgraded to be able to kill any kind of Zombie (Panzersoldats included) in one strike but it only lasts for one minute and then the player has to upgrade it again. This can be done by being the one who gives a Panzer Soldat the final blow (the one that ends up killing him, doesn't matter which weapon is used).
First, the player must acquire the Ragnarok DG-4 and put them down as a trap behind the clock.Then hold the use button on a switch inside the clock tower to start the clock. Then the player who wishes to obtain it must stop the clock at 9:35, as a reference to Group 935. After this step is completed (this is shown by lightning hitting the bell tower alongside a completion sound), the player must wait for an anti-gravity sequence and wall run onto the circle to the right of the entrance of the Pyramid room. To go back in time, wall run over the glowing blue circles around the room when the lightning hits the bell tower. After this the player will be sent back in time, and will be able to obtain the Plunger, which is next to the desk, opposite the safe and to the right of the teleporter. After the player is sent back to the present, they will have obtained the plunger as a melee weapon.
On a table in the Control Room there is a newspaper talking about a parade in London. If Zombies are killed using the upgraded Plunger near the newspaper, this one will start changing. It will first say that there's a possibility of zombies roaming around the city but is unclear and later it will say that there's a large outbreak occurring and that the Parliament is setting up a meeting to confront the crisis. (Note: The easter egg has been erased in one of the April updates).
The clock is automatically set to 1:15, referring to Element 115.