Found a method to get a S rank on the "kill two operatives" side op. The method requires a previous A rank to unlock the sniper rifle at spawn.
Grab the sniper rifle. Right from the start tranq the first guard you see on the rocky beach on the left side, move his body close to the road so a person driving a jeep can see it. Wait for the jeep to stop and both people to get out. Tranq both of them. Then tranq the guard patrolling around the old POW prison camp. Drive the jeep to the POW camp, break in, take both POWs and put them in the jeep.
Drive on the opposite road, not the one you started on and park 3/4 up the hill/road. Get out and move forward enough so you can see the big guard tower overlooking the supply ruck entrance. Take out the sniper rifle and zoom in enough so you can see one of the targets in the tower. Line up a headshot and cap him. Run directly forward through the camp ahead until you reach another road, zoom in with the sniper rifle through the chain link fence and you should see the second target with a couple of dudes.
Get a headshot, run back to the jeep with the POWs, escape by jeep. The final mission time should be under 5 minutes and you should squeak by with a S rank if you only killed the two targets.