To successfully pass the mission, follow Leo Garcia without being noticed, then disable him. To earn a commendation, you must suffer 50% or less damage with a 90% shooting accuracy. For the first part of the mission you have to be very careful not to get too close or too far - keep an eye on the distance meter. If he stops moving for a while just be patient and wait for him to pull away. After the car makes the drug deal chase after him and disable his car. Also, try to take out Leo's vehicle as quickly as possible so you will have enough time to go back and get the evidence - you only have a short period of time before you can recover the evidence. Again be careful not to hit any civilians with gunfire or else fail the mission.