Secret Achievement: Complete the penultimate agency mission in the game. It is called "Into the Den", as per the achievement's namesake, and involves you infiltrating the president's palace in the southeast corner of the map.
Before starting, I recommend having a machine gun fully upgraded or at least Level 3/4.
The first part of the mission has you invade three posts, each with an anti-aircraft gun. Real simple, kill the guards, destroy the AA turret. There is a cable car that goes from stop to stop you can take it or if you are psycho like me, jump and parachute climb to the next. After each AA gun is taken out an engineer can land to help get the base open.
After all three AA guns are taken out you can get in the base by activating a switch to open the Mount Rushmore-esque entrance. Run across, a cutscene opens, and now comes the hard part: escaping the base. Focus first on shooting down the helicopter (with the upgraded Machine Gun I recommended entering the mission with). Don't bother trying to hijack it, for you will likely get put down by the hordes of enemies shooting from below before you can start piloting it. Once the heli is destroyed, make your way down to the center taking out any and preferrably all enemies (shouldn't be too hard), activate the switch, and quickly grapple to the entrance way you came in on. I mistakenly believed you had to grapple to the roof and escape out the top but I obviously didn't make it.
Once outside, the achievement and sixty gamerscore is yours, but beware the seventh and final mission begins right where this one ends so once you enter this mission you are in the endgame.