Speed is key to securing an S rating. Choose a starting point close to one of the two commanders travelling to the meeting point - ideally the one to the north, as extracting his companions as well offers a score bonus. If you can send all four back to Mother Base while they're still in the vehicle, all the better. You can then gallop at full speed to extract the second commander before he makes it to Wialo Village, then reach and recover the third at the meeting place before you leave the mission area.
