You'll be battling a chopper soon, so take the FIM-43 and Supply Marker
You start the mission with the guards already on alert and knowing where you are. There are many aggressive enemies, i.e. if you wait, they will come to you or throw smoke, stun and frag grenades. If you sit still you'll be overwhelmed. This makes it one of the more challenging normal (i.e. non mech battle) missions.
If you don't mind killing people, take the M60 and sniper rifle too. If you do this, you'll need to equip the Battle Dress uniform. If you're aiming for a zero-kill run, take the Mk-22 and Mosin Nagant. They don't work against drones, so you'll have to use the FIM-43.
If you want to S-rank this you WILL need rubber bullet shotgun, once you have obtained it all you need to do is just run and shoot, don't bother about actually knocking them out, only shoot them just to make them fall over for 5 seconds or so. Also using the mounted gun against the heli, wont kill that soldier flying it, get around 7 or so minutes for a s rank
Next: Peace Walker Battle 2