In this stage the WP Bonus is harder to get. It's a good idea to put yourself on Akyu's house to keep yourself safe while forgetting about the bonus.
The reason for the Bonus to be harder is that Rumia will start moving only after the third turn.
For the first turn, you'll have to deal with the the barrages deployed by the Blue Fairies. Since you can field Youmu on this stage, it's a good idea to use her to get rid of those. The Kedama is somewhat along the center of Rumia's barrages, so you won't be able to attack it right away. However using ranged weapons isn't useful at all...
This is because Rumia's Barrage has the added effect of halving the accuracy of shooting weapons.
Thus, it's a good idea to leave Rumia to Keine and Youmu only.