An easy achievement but there are somethings you should know before starting,...and knowing is half the battle.
First off, let's begin by selecting the "Athens" mission on the "recruit" difficulty setting. You will have infinite ammo, so feel free to go cowboy. Shortly after you are introduced to takedowns, there are two more thugs that need dealing with. Here is where you have to be careful, because after you dispatch them, you will automatically pick up one of the goons weapon, the MP5. Make sure not to fire it and just switch back to your pistol. Complete the boat chase and arrive at the Athens resort.
Immediately starting at the Athens resort you will begin with the MP5 already equipped, make sure to hit the Y button to change weapons to dispose of the bodyguards outside the hotel. Finish the driving sequence, and the achievement will pop right around when the silly Greek general is talking about fireworks.