First thing to do, build Ragnarok DG-4 and Ancient Bow
This guide is same as how you can get free gobblegum. Once you have all those weapon. Head over to the Clock Tower all the way to the top. Look on the wall where there is a mark, then shot using bow. After that, wall will be open and the clock mechanism will reveal plus the part for fire bow. Next, you need to use your Ragnarok DG-4 and place it down behind it. Now, go all the way down near before exit door and you will see a small lever that you can activate by pulling. After you pull it down, the clock will be alive and ticking. Next, you need to stop the clock on exactly 9:35. On this next step, you need to activate the anti-gravity. Then, run on the wall where there are a round symbol. You need to run randomly will stepping on the symbol and if you did it correctly. You will be teleported. Then, Head over to the table near teleporter machine, press action button once you see a plunger because this takes only a couple of secs and it will bring you back in time. Note: If you are playing co-op then all of the players you are playing will have this. To make plunger one hit kill, you need to kill panzer any of your choice weapon but this take couple of mins of use. To try it, melee in the air and your plunger will have some effects around it. Its really useful if you are playing higher rounds because this can kill panzer in just one blow of melee.