To recruit Celebi, you must go into Purity Forest with no items, no money, not even a held item! And if you dare to bring any items, they will be destroyed upon entry! That being said, the only real preparation is to choose the one pokemon you will bring into the forest! Charizard is my suggestion because his heat wave attack can take out an entire Monster House in one blow! You can possibly find a Monster House on any floor, but you should especially watch out on floors 29F, 46F, 64F and 86F! On the plus side, You can find Kecleon shops on any floor but they're common on floors 7F, 23F, 38F, 55F and 75F! All pokemon will enter the forest at level 1. You will find Celebi waiting on 99F. There is no boss fight here. Celebi will ask you if you want to recruit it. If you answer "Yes" It will give the healing forest friend area and join your rescue team! If only the other 98 floors were that easy!