1) Set a team of 3 Pokémon.
2) Set the tactic to Go the Other Way. Your teammates will need to be tough enough to take care of themselves.
3) Equip your main character with the Satisfied Looplet.
4) Go to options and turn the dynamic camera off to save time.
5) When you enter a new floor, hold A+B to run a lot faster.
6) As soon as an ally gets to the stairs, press L to switch to them and climb up.