This unusual sidequest can be started at Humilau City . From the Pokemon Centre, head east then north towards the houses on top of water. Make your way to the central house and go inside. Speak to the Rich Boy and he’ll request you to ring Celestial Tower’s bell for his newly bethroved.
Beware that they get progressively stronger the higher you climb.
Once you’ve rung the bell on the 5th floor, head back to the Rich Boy. For your efforts he’ll give you a… Revive ? Um, thanks? Well, at least you earned some battle experience.