Buy the Thermite Grenades in the Undercroft to the Right of Mule Kick.

Throw a Thermite Grenade at the 3 Wooden Piles Outside the Map in Quick Succession

• Meteor Crash Site Above the Bunker Door.
  
• Running from Meteor Site, Go towards spawn to the Left, and its outside the Metal Fence. (You have to Jump to See it)
  
• Running from Meteor Site, Go towards spawn to the Right, and its outside the Yellow & Black Barricade.

Once All 3 Wooden Piles are Lit at the Same Time, Get a Kill on the Explosive Sign next to the Car at the South Evac. (Opposite the Bunker Door)

This will Spawn a Blue Circle on the Ground. Get a Bunch of Kills Inside the Circle, and the Wonder Weapon will Spawn for you to Pickup.