The achievement asks you to finish the Archives level in under 15:10 on Agent difficulty or higher.
I would also like to add a few tips here of my own.
-Set the difficulty to Agent. Trust me, it will save you a lot of time and grief from the higher difficulty settings, unless of course, you want a better challenge.
-AK47 is indeed a great choice, but make sure you have a shotgun with you. One-hit kills will do wonders instead of a dozen bullets from your AK47.
-Gun down many enemies as fast as you can. In some situations, you'll have to rely on cover to regenerate your health. Use quick scoping and take them out fast.
-When it comes to the elevator section, immediately start having the elevator come back up, then pick off enemies as best as you can. Make sure you stay by the elevator. When it opens, at least two guards will come out. Dispose of them before running in and closing the door behind.
-Same thing with the Tank. Press the button, pick off the enemies again. Once the tank is available, do not worry about any of the enemies. Just rush towards it and get in the vehicle.
-Like Ultima22 said, don't worry about any of the secondary objectives. Focus on the primary and move your ass to the end as fast as you can.
In doing this achievement, I earned it with two minutes to spare.
It can be done!
Good luck to you all!