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.
Well, why not? Head to Celestial Tower, ideally by Flying from Mistralton City and heading there via Route 7. Now you've already been through Celestial Tower before, so you should know what to expect. However, in front of each stairway, there's a female trainer that won't let you by without a battle...
Beware that they get progressively stronger the higher you climb.
Floor 1 Battle: Waitress
Floor 2 Battle: Nursery Aide
Floor 3 Battle: Clerk (F)
Floor 4 Battle: Maid
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.