So the easy way to do this is to create your own contract, play it once to deliberately get a poor score, then play it again to get a better score and unlock the achievement.
The simple approach: create a new contract at the ITA training facility, mark the guard patrolling the front entrance (Luca Wood), shoot him in the head, then quit. Save the contract.
Immediately play the contract you created (under "My Contracts"). When it starts, deliberately wait about 30 seconds or so, hiding behind the vehicle, before killing the guard and finishing the level.
Immediately after this is over, replay. This time, kill the guard immediately and exit the level. Your new score should be better than your first one due to the time bonus and you should unlock the achievement.