Easy "S" ranks
Use the following trick to get easy "S" rank unlocks. Play Wesker's Mansion Mission (Rebirth). Play the first scenario repeatedly. Once you find all three files, all you need to worry about is time and critical hits. It is also a short mission. Playing it over and over again will not be a chore. Once you get the three files, getting an "S" rank all the time will be easy. The best time to get all your critical hits is after going up the steps. Once the zombies begin moving, aim your crosshair until the red circle blinks, then fire for an instant critical.
Every time you see a spawn of small enemies, throw a grenade to kill almost all of them. Get a number of enemy kills at the final score to get an "S" rank on that category.