I got an 'F' -- How can I improve my grade?

I was working on the "All I want for Christmas is Sewers" mission in Rusty Hearts, (Canals B3, Very Hard) and I got an 'F'. How can I improve my grade? I got hit a few times, and I got what I thought was a huge combo (50ish?) Is there something else I need to do?

Your rank in a dungeon is affected by 3 factors:

Time

Self-explanatory. The longer you take to clear the various rooms, the lower your rank will go.

Style Points

The more Style Points you get, the better your grade will be. Ways to rack up Style Points include:

• Perform high combos: Performing several attacks in succession and comboing your Skills with your normal attacks will help increase this quickly.
• Overkill: Hitting corpses that are already dead will cause Overkill, as will doing more damage on a killing blow than life a monster has left.
• Grab Finishers: Killing monsters with grab skills will give you bonus points, though apparently it also lowers the speed you kill/clear a bit, so you'll have to weight the benefits for yourself.
• Chain Kills and Rising Hits: Slaying lots of monsters one after another will get you Chain Kills and doing air combos/juggling will get you lots of Rising Hits.
• Blocking: Timing your blocks to do a Just Guard in order to counterattack. This is also good because the third factor is...

Getting Hit

The more you get hit, the lower your rank will go. It seems that the number to avoid is 16. After being hit 16 times, your rank will go down a level. So if you're at rank SSS and get hit 16 times, you'll go down to SS, another 16 hits, and you'll go down to S (excluding other factors). According to some, it may even be impossible to get SSS if you have massive Style points, but were still hit 16 or more times.

Some dungeons also have requirements where you must have a certain amount of Style Points by the time you get to a certain room, or your rank will drop. The playerbase has speculated on the formula a bit, but there seems to be either an element of randomness or there's just not enough information yet.