Normally, you would have to fly into the barrels of the giant cannons in the Navarrone Valley to destroy the targets at the end. However, due to a glitch, you can sometimes lock onto those targets from the outside of the cannon and your shots will go right through. This seems to work best with a plane that has Multilock-on Ground Missles, such as the MX5000. (It's hilarious to see the Python Units' reaction if you take out the first cannon before they can.)
Taken with permission from Flying Omelette.