Recruiting Pokémon in Pokémon Mystery Dungeon Rescue Team DX usually boils down to basic RNG based on your level and a few other key factors within the game’s code, but recruiting Mythical and Legendary Pokémon is a little different.
Celebi is one of several Mythical Pokémon that will join your team automatically if you can complete a very challenging dungeon after meeting some pre-existing conditions to unlock them.
However, unlike Jirachi, Celebi is a relatively easy recruit once you get past the fact you need to complete 99 Floor challenge dungeon to get it. That means your level and stats are reduced back down to Level 5, even if you are fully evolved, and you then need to go through a full dungeon to reach Celebi.
The dungeon in question is Purity Forest, which you unlock by completing Pitfall Valley and the Latias storyline. You can’t bring any teammates with you, and your inventory is reset along with your level.
However, despite the fact you can’t bring any Rescue Team members with you into the dungeon, you can recruit new Pokémon while traveling the 99 floors, and they will assist you throughout the challenge. You will encounter mostly Bug and Flying-type Pokémon early on, but later, you will see Steel-types like Metagross and other powerful creatures too.
You won’t even have to fight Celebi at the end of the journey, so don’t worry about grinding too hard since there isn’t a final boss. All you need to do is accept its request to join your team, and then you will clear the dungeon with a new Mythical Pokémon in-tow.
You will also keep every item and all of the money you earned throughout your time in Purity Forest, so feel free to go out of your way to grab some useful stuff if you see it too.