I've been doing that. Doesn't work. Could you give some more detail, please? I'm missing something here...
First thing you do in the morning after you have won the swordfight is going to the library and talk to Elsa.
Wait until next morning, invite the girls and after Elsa shows up go back to the library and talk to her again.
Wait again next morning and reinvte the girls, this time Elsa will show up asking to be appointed as SHITT (Don't ask ;-) ), right afer that, talk to Martha, give her the sword and wait until Cassandra shows up in castellan's quarters... This time, since you gave a new role to Elsa, she will not appear at night with the Master.
Next morning, if you made it before the fifth day just invite the girls again and cassandra will show her panties and that's it.
It's important you don't waste time to reach the swordfight with Cassandra or Meridiana will show up anyway....