40, I think. If 33 is the one I think it is (C2W Subsistence), it's a cakewalk to S-rank if you destroy the anti-air radar at that communications outpost in free-roam, select the LZ in the outpost for the mission, then use the helicopter's minigun to shoot all of the radar boxes while you're flying in. If you're fast enough, the mission will finish before the helicopter attempts to drop you off.
Note that you'll probably have to upgrade your helicopter's armor by 1-2 levels so it won't explode from all the gunfire it'll take since you'll be flying right where everyone can see you.
I'm not looking forward to working on the mission tasks for that one, though, or for any other Subsistence mission.