In Die Rise, kill 5 zombies with a single shot from the Sliquifier.
To build the Sliquifer you have to find the 5 parts in the upper and lower floors of the power room area, the spawns are very random. The pieces can be found on the floor and on desks, most of the pieces blend in well so be sure to cover everything to find the pieces. Once you have found 1 you want to go to the lower floor, here you will find a desk in which you can build the Sliquifier. Find all the pieces then you'll be able to claim the gun.
Gas Canister - It is either directly next to where the Sliquifier is built in a cage, or directly next to the power switch on a table.
Boot Heel - Near the top of the staircase, next to the television. This part never moves.
Dish - It is either on the barrel next to where the debris was bought, or it is in the room upstairs with the showers and washing machines on a wooden stand.
Corkscrew - It is either in the power room on the tables by the elevators, or by the fridge near where you build the Sliquifier.
Once you have built it simply take it and it will be added to your inventory or replace the gun you are holding at the time. Be careful though, if you run out of ammo you will only be able to get the gun again from the mystery box. As soon as you get it you want to make a train of zombies (have a group following you). When there is 5+ simply shoot the gun once into the group. The gun will kill everybody it touched and also cause a chain reaction, so if a zombie explodes near an unaffected zombie it will kill them too.