To unlock this achievement, you need to complete any 6 of the level trials that are available while playing on the Agent difficulty. A list of 6 level trials on the Agent difficulty can be found below;
Goldfinger - Auric Enterprises - Target: Shoot the 7 targets.
Goldfinger - Fort Knoxx - Elimination: Make 100 eliminations with the AK-47.
O.H.M.S.S. - Alpine Valley - Elimination: Make 7 eliminations with the Stauger.
O.H.M.S.S. - Blofeld's Lair - Target: Shoot the 7 targets.
License to Kill - Refinery - Specialist: Make 40 eliminations with the Walther P99. Making eliminations with other weapons will fail this trial.
Die Another Day - Ice Hotel - Specialist: Make 50 eliminations with the STK-21 Commando. Making eliminations with other weapons will fail this trial.
You will know when you have completed one of these trials, as it will appear in the lower left corner of the screen upon completion. You can also see which ones you have completed by going to 'Mission Select' form the main menu, and as you highlight a mission, you will see on the right hand side of the screen, a list of the trials underneath each missions details with a box next to each. If the box has a tick in it, then you have completed it.