The Classified Intel acquisition side op in Metal Gear Solid: Ground Zeroes is incredibly easy once you know the location of the cassette tape you need to retrieve. This guide will show you how to get through the mission without touching a single guard and also bagging a handful of achievements/trophies in less than 10 minutes.
The mission starts with Snake in the back of a truck, and you are going to stay in this vehicle until it enters a guard gate. Once you hear the sirens of the gate opening, jump out the back of the truck and head left, climb over all the piping and stay along the right-hand wall.
A guard will walk from right to left and stand guard for a moment behind a metal freight. Mark him with your binoculars before he goes behind the metal crate, as the second he turns around you're going to head left and up the ladder. Don't tranquilize him, as the guards who head to check the security camera you're about to destroy will notice him unconscious and it runs the risk of adding time to your run.
Run along the platform and up the next ladder, equip your silenced automatic AM MRS-4 rifle and look up-left before entering the watch tower to take out the security camera. The guards will speak on the radio and begin sending troops to check it, but you'll be gone before they get there. Grab the cassette from the table and head out. Head down the first ladder and jump straight over the ledge onto what looks like a book case. Jump back into the truck and lay down. Crouching runs the risk of being spotted by guards behind the truck and above (you'll notice in the video Snake is almost spotted by the guard heading to the watch tower to check the security camera, if this happens to you, just lay down and you'll be fine).
It'll take a minute or two for the driver to get back into the truck and drive out of the base, so just sit back and wait. You've successfully completed a side op, as easy as that!
Achievements and Trophies
By exiting the base on the truck you'll receive the "Infiltration" achievement, worth a handy 50G or a bronze trophy. If this is your first side op, you'll also get Genesis for 50G or bronze and 'Skilled' for obtaining your first S-rank (100G, silver trophy).
If you're still having trouble, check out our video below of a complete speedrun of the mission.