In the Castle Area, two screens before Sekibanki's head:

1. With Aya, dash and jump to the right end of the platform, and use Aerial Slash to take out the top wizard Fairy
2. While Aya free falls, use another Aerial Slash to take out the middle wizard Fairy
3. Switch to Momiji and use a Robe Slice to take out the bottom Fairy
4. Return to the top zone and repeat

This method when executed perfectly nets 3 kills every 6.5-ish seconds. Optimally, you kill one Wizard Fairy in the large room, kill 3 fairies on the way to Sekibanki, and do this loop 32 times, and will take around 3 minutes 30 seconds if done perfectly. Reference video:

Current route for Platinum Trophy will involve sequence breaking to get to Daiyousei as fast as possible, then warp back to Castle Area, get Sekibanki's head and farm until 100 kills.