If you want to win this battle legitimately the real enemy here is Astraea's bodyguard Garl Vinland. The knight blocks the path leading down to her.
Do not attempt to just drop down or you will be inflicted with plague and swarmed by small enemies. Running past Garl Vinland on the other hand will make him follow you and attack, so just take care of him first. He will stay in his place blocking the path until you get up close. His attacks are easy to parry, so just parry and riposte to kill him quickly. Afterwards you may continue down and attack the maiden. She will not move, but
occasionally heal and do a magic blast hurting you and throwing you down, but with some healing you will quickly reduce her HP to 0. Be aware you will be inflicted with plague standing down there so wear a ring of plague resistence and heal the status ailment with Widow's Lotus if neccessary.
You can also cheese her by never going down and fighting Garl, but instead shooting Astraea with arrows from the top. Doing so will grant you an additional trophy.