Instructions
Start a campaign on the ninth level -- "The Package" -- in either solo or co-op and set the difficulty to "Legendary."
Make your way toward the tank on foot. Move cautiously, paying extreme mind to your surroundings. Move slowly around corners and take out the turrets and ground forces before they have a a chance to fire at you, or have a co-op partner take them out in a hijacked enemy vehicle or on foot before advancing with the tank.
Shoot down the banshees flying over the artillery guns as you make your way up the hill. Bombard the enemy artillery guns with rounds from the tank and blow up the three ghosts lurking around the corner.
Take out the wraith nearest your position and the plasma cannons in the mountains from a safe distance (take cover behind the rock and fire or have your teammate use the designated marksman rifle to pick them off). Move up slowly to the tank and take out the mounted guns, the ghost and any remaining enemies.
Remain in the tank and cross over the stream and uphill to earn the "Tank Beats Everything" achievement.