Recruiting Pokémon in Pokémon Mystery Dungeon DX Rescue Team usually comes down to base RNG depending on your level and a few other key factors in the game code, but recruiting for Mythic and Legendary Pokémon is a bit different.
Celebi is one of the many Mythical Pokémon that will join your team automatically if you can complete a very difficult dungeon after fulfilling certain pre-existing conditions to unlock them.
However, unlike Jirachi, Celebi is a relatively easy recruit once you get past the fact that you have to complete the 99 story challenge dungeon to get it. This means that your level and stats are brought down to level 5, even if you are fully evolved and then have to go through a full dungeon just to reach Celebi.
The dungeon in question is Purity Forest, which you unlock by completing Pitfall Valley and the Latias storyline. You cannot bring teammates with you and your inventory is reset with your level.
However, despite the fact that you cannot bring Rescue Team members with you into the dungeon, you can recruit new Pokémon as you travel through the 99 floors and they will help you throughout the challenge. You'll mostly come across Bug and Flying-type Pokémon at first, but later you will also see Steel-types like Metagross and other powerful creatures.
You won't even have to fight Celebi at the end of the journey, so don't worry about grinding too hard as there is no final boss. All you have to do is accept his request to join your team, then you'll clear the dungeon with a new Mythic Pokémon in tow.
You'll also be keeping all of the items and money you've earned throughout your time in the Purity Forest, so feel free to go out of your way to pick up some useful stuff if you see it, too.
