Instructions
Select "Mission 2" from the mission selection screen.
Press "A" on the command center building. Select an empty section and select "Build Vehicle Depot." The depot will allow you upgrade the Scout vehicle to a Warthog capable of firing bullets at enemies and destroying the methane tanks.
Build a combination of Warthogs and infantry units until your reach the population capacity. The more units in your army, the easier it will be to destroy any Covenant forces surrounding the methane tanks. Once the population capacity has been reached, press and hold "A" to select all your units.
Press "A" to send your units to undiscovered areas. The black areas indicate that you have not explored the area, and since methane tanks are scattered throughout the entire map, you will have to explore each black area throughly. When you discover a methane tank, highlight it using the analog stick. Press "X" to fire on the tank.
Ensure to check every area of the map for tanks. Continue searching the map until you receive a notification titled "Endless Fun" at the bottom of your screen to verify that you have earned the achievement.