This is done for completing the penultimate...err, mission, in the game. You must do this mission on "Hard" difficulty, but be warned, you will not get:
for completing it this way.
This mission sees you split up from your squad and you have to go and rescue them on your own (don't forget to use the auto-injector when you're in trouble!), after doing so, you have to locate a man named Serdar, and the mission ends. This mission is A LOT longer than I explained, but if you keep following objectives it should explain itself.
There are five Gold Crates in this mission and the collectable weapons are M95, XM8-LMG, AK74Su, and a VS5, I believe.
You can also get this achievement on this level as well:
Good luck!