The hardest achievement in this game by far. Now of course if you are a pro gamer and can achieve this the way it was intended hats off to you. But some of us don't have the time to grind it and get good at the mission or we lack the necessary skills and reflexes to be as fast in the first place. Well there is an easier way to get this achievement:


1. Load up Mile High Club on the recruit difficulty
2. Play the mission until the part where the last terrorist opens the door and is holding the hostage
3. Pause the game and Save & Quit (don't exit to windows, go back to the main menu)
4. Go to the mission select screen and start up Mile High Club on Veteran
5. As soon as the mission starts loading Alt Tab out to windows (or Alt+F4 to close the game) or use the task manager and force close the game (try to do it as fast as possible after it starts loading)
***DISCLAIMER***
DO NOT MANUALLY SHUT DOWN YOUR PC DURING THIS STEP BY HOLDING THE POWER BUTTON, IT MAY CORRUPT YOUR SAVE
6. Start the game up again and RESUME the mission
7. If you closed the game fast enough and did everything else correctly, when the mission loads up you should be on the part at the end of the mission where you have to shoot the final terrorist holding the hostage but the difficulty will be Veteran.
8. Finish the mission and the achievement should pop


I got it this way and technically it is an exploit but if you really don't want to stress your nerves trying to get this achievement I highly suggest doing this method as it's faster and easier.

Disclaimer: I did not come up with this guide, I found it online and it's originally for the console versions of the game. I'm just spreading word of the solution here. All credit for this method goes to FPSDeath's guide for it here: https://www.trueachievements.com/a225338/mile-high-club-achievement