You have to beat a Boss from the DLC on Expert Mode without losing a life, doing 3 parries or more & use your Special 6 times (one time full & 1 simple). You also need to beat the boss as fast as you can.
I recommend Ms Chalice as she start with 4HP and you can still have S Rank even if you lose a life, I also recommend the Charge Shot as it gonna make you beat the boss faster.
For the Boss choice, there are two of them that seem to be the easiest way :
-Moonshine Mob : He's the first boss, ton of parries & relatively easy first two rounds. For the second round with the lady & her radio, use the invicibility dash of Ms Chalice to avoid the red flows, you cans stay at her right or left & use your special to get her done quick.
-Mortimer Freeze : This one is tricky, but if you play well, you'll be able to dodge a lot of his attacks with the invincibility dash. I found him less hard to finish with no hit but there is not much parries as the first one. This one is for challengers, but easier than other bosses.
Good Luck ! And sorry fo my English :p