As the Regi puzzle of this generation, it can be confusing. When you first enter, get to the middle pillar in as few steps as possible. Talk to it, and it should move for you (it is recommended that you use the left entrance). On the second floor, use
Strength Flash right when you're near the pillar, and go up. On the third floor, use
Flash Strength. And then you're done! The key here is to take as the smallest amount of steps possible before the torrent of water kicks you out. If you need a map, use
this one by Mew Jadester.
Edit: In response to below: Yeah, oops. Use Flash on the second floor, and Strength on the third. Thanks for the correction.