1- You will need atleast 1 lvl100 pokemon to Challenge him. Reward: A lvl25 Shuckle (Can be shiny)
2- There are 29 floors, you have to pass them all... and yes u need Membership to enter into chamber.
Heres a guide on it: https://pokemonrevolution.net/forum/viewtopic.php?f=89&t=3104