Figured out the trick to getting it. Deliberately get eaten by the Faults once per room, and then kill all of the enemies that spawn each time you enter the underground section. It should get you enough Style points to finish S ranking it.
You ever figure out a good way to get an S rank on Mission 14 in Son of Sparda? Missions 1 &14 are the only ones I still need, but I've tried each several times and am out of ideas (aside from generically aiming for the "No Damage" bonus).