This mission is really easy to complete if you know what to do and do it carefully. You can get an S rank by just getting the fake casette tape and escaping without even seeing the 2 main characters in the mission. The mission will take around 7 minutes since you have to spend 3-4 of them hidden in the truck.

To begin with, just lie down on the back of the truck so you are not spotted. You can stay crouched but you could be seen this way. Wait until the truck enters the main building through the big gate and step out of it. Hide somewhere and tranq the driver when he gets out of the truck. You don't even need to hide him if you are quick enough.
Continue through the alley to your left and quietly tranq the guard. Then, picklock the gate to the left.Look for the open box and disconnect the power supply so that the security camera hidden in the watchtower doesn't spot you. This will last for enough time for you to complete the mission and not really worry much about it any more. Next thing you want to do is climb the ladder to get to the watchtower.

Originally posted by Big Boss:

I suggest you throw a mag to the right of the guards so that they look in that direction, since they may be able to see you after you get the cassete tape. I'd recommend using the long throw by holding down the throw button

Finally, get the cassete tape and jump down to your right, where the truck and the unconscious driver will be waiting for you. Kick the guard to wake him up and quickly get into the back of the truck again. He will be surprised and alert the rest of guards but they won't know it was you or where you are, so he will just get back on the truck and drive you out of the base.

I know that this is a very simple guide but I wanted to try the Steam Community Guides editor and I'll be happy if it helps someone. I plan on doing a Spanish translation as well. Leave a comment if you have any doubt or suggestion!