This took me several tries to do. Here's what I used to make it happen.
Important: You must play this solo, if you do this cooperatively, this will be a lot harder
Gobblegums used:
- Anywhere But Here (purple, requires level 10)
- Stock Option (green, default)
- Firing on All Cylinders (blue, requires level 23)
- Sword Flay (green, requires level 6)
- Always Done Swiftly (blue, default)
At the start, immediately rush to the wall to the left (ignore the trials, that is not the focus of this challenge) and pick up the RK5 pistol. It is a burst fire pistol, will last you until Round 4. At round 3, pick up the Quick Revive (500) and open the first door (500). Pick up the KSM-12 (shotgun wall buy for 750) and move to the barrier on the edge and shotgun enemies entering the door.
Once Round 4 is done, go upstairs (not to the room with Juggernog) to wall buy a Kuda. This weapon will last you all the way to 21. Every so often, you may get lucky with Death Machine drops, Nukes, or Insta-Kill powerups, those will substantially give you more time.
DO NOT ABSOLUTELY PICK UP CARPENTER POWER UPS (Hammer) IF YOU SEE THEM! Those will slow down your progression through the stages much more heavily.
In Round 6, turn on the power and try to find the shield pieces on the map.
Pieces are found in:
- The Operations Bunker on the bottom floor (can also be on a truck door outside or in the Juggernog room)
- The Infirmary, various locations
- The Armory, various locations
Knowing where the parts are is essential for this, so you can save yourself in a fine touch.
After building the shield, you next need to wall buy the Argus (found in the Infirmary) then take the Elevator trap to the Armory. Position yourself in the corner and use your Kuda to mow down zombies. In Round 9 or 10, go to the Tank Factory and get the Double Tap Root Beer, it is core for this run. Around Round 13, go through the burned train to get Speed Cola.
Two waves between 8-11 and 16-19 will be Valkyrie Drone waves. You need to shoot them with the Argus shotgun in the chest. The arms may sound easier but Double Tap gives you a huge damage boost against the drones.
If you go down at any point, you likely will not complete this. If you get surrounded, either use the "Anywhere But Here" Gobblegum if you have it, or pull out the shield and fire a fireball at the zombies to save yourself.
To get this achievement, you must beat Round 20 in under 32 minutes. I did this in 27 minutes but it took several attempts. Some of this is RNG based with Death Machine, Insta-kill, Max Ammo, or Double Points drops.
I used this video as a reference. If you level your Kuda and Argus weapons, this will be a lot easier. Good luck!