I finished CF 1 and 2. In CF 1 in mission 8 phase 2 (if I recall correctly) it gets really hard. It's the mission where you need to get to the gun in the middle island and you can't destroy the bunkers with normal rockets and grenades. If you beat that one you beat one of the most difficult missions in the whole game.
Later it gets more easy with choppers you can fly and other cool stuff. There are some more difficult missions after mission 8but not that many.
