① The Three Fairies hide their forms and sounds, then secure a deck brush and bucket (water).

② Cirno and Daiyousei then meet up with Rumia (and the three hidden fairies). Once they run away from the Chief Maid until they're close to a wall, Rumia covers them all in darkness.

③ Hidden and silenced, Cirno, Daiyousei, Sunny and Luna come out from the darkness. Using Star's ability to confirm the Chief Maid's location, Rumia attacks. (The Chief Maid will probably dodge and escape out of the darkness.)

④ Sprinkle water (silenced) on the ground near the Chief Maid.

⑤ Once the Chief Maid touches the water, freeze it.

I hope this explains it so it's easy to understand...

Why do we escape to a wall?

So the Chief Maid will get the idea that "They're all still in there" even when she can't see in the darkness, since there's no escape route behind us, see? Also, even if we can't see in the dark either, we can easily get through and escape by following along the wall.