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:
Self-explanatory. The longer you take to clear the various rooms, the lower your rank will go.
The more Style Points you get, the better your grade will be. Ways to rack up Style Points include:
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.