Infiltrate the Crater Base

Los Cantos

You'll have to cross a broken bridge, with the majority of the top section broken. Locate the nearby stairs and descend to the lower level. This area is swarmed with enemies, so you'll need to CQC or tranq them one by one. When ready proceed to the second stair case and up to the next part of Los Cantos.

You'll be at a rock face ridge, and the enemies here will have helmets. This gear advantage will protect them from a single headshot, forcing you to waste double the ammo from your MK.22. A beter option is to CQC them and proceed to the next zone.

Fuerte La Ladera

You'll finally arrive at a real fortress with not only guards, but turrets as well. If you've constructed a sniper rifle, now is the time to make use of it. If however your R&D was unable to develop one, you can wait for each of the soldiers to descend to lower terrain and CQC them each individually. When all of the enemies are dealt with, move north west to enter an interactive cutscene.

While your objective is to locate the truck with license plate 64779, if you take the time to examine the other trucks, you'll be able to spot some amusing cameos. Looking inside truck 63824 will even reveal Hideo Kojima, the game's creator, who will in turn join your army.