Question: Is it possible to get an 'S' rank in Resident Evil 6? If so, what are the general requirements? I know some will probably be subjective to the level (time and enemies) but what about accuracy and deaths? is there a set rule for an 'S' rank? (assuming it is even possible)

This was the case in most of previous games. So far my best is an A rank, and I suppose the closest I have to knowing if an 'S' rank is possible is that my number of deaths was at 1 (which gave me an 'A' rank) and I wonder if 0 deaths would be an 'S' rank.

Anybody got anything better than an A?

Answer: 
I don't think the time matters that much. The most important factors are how many enemies are killed and especially the accuracy; how well you aim at the enemies. You shouldn't do "straight bullet"... missing shots will definitely decrease your chance to get an "S". Try to kill them with no misses; critical hits are good, but the upper body is better for a novice!
Simple answer, you just need an A rank in the 4 categories. You can't get an S in each category, as obviously a zero death would count as an S, but if you get a B in any category, you'll get an A or less instead of the S.