Celebi in Crystal

Warning

This method is highly experimental!

Gen 2 RNG is still in its early days. Expect to need multiple attempts!

Tools

Save in front of the Celebi shrine. This lets you reset if needed.
Press A to interact with the shrine and stop when you see Player put in the GS Ball.
Open PokeReader to the RNG screen and wait until Finding ADIV Index and Finding SDIV Index are gone.
Pause PokeReader with L + R then enter the info from PokeReader into the RNG tool below. Press "Generate".
Unpause the game with R until near the target advance.
Pause then advance with L one at a time.
Stop when the "Advance" number in PokeReader matches one shown in the tool.
- (Optional) Make a save state if you're on an emulator.
Press A to accept the Pokemon. Do not press anything else until Celebi appears!
If you got your Pokemon, congrats! If not, keep trying!

This is unstable! Expect it to take many tries!
Only use L + R to pause and advance in Gen 2, not Start + Select. Start + Select will mess up the RNG!
Do not press any buttons in between pressing A and seeing Celebi.
- Switching screens or pressing buttons during this time will mess up the RNG!
The 3DS registers button presses instantly, but the Game Boy takes a few frames.
- Pressing A too quickly might unpause the game without starting the encounter.
Don't hold A too long. Press it slightly longer than one frame.

Advance	State	Shiny	Max DV
Advance	State	Shiny	Max DV
No data

Now you can use Transporter to RNG your Celebi to have perfect IVs!

All the people who have looked into Gen 2 over the years: Admiral Fish, EzPzStreamz, Zep, Real, Amab, wwwwwwzx, Vlad, Bambo_Rambo, Lincoln, Shiny_Sylveon, and Zaksabeast.
Zaksabeast for coming up with the current RNG method and building the tool.
Shiny_Sylveon, El Terapagos Mexicano, and Lord Timothy on Discord for helping test.
Hakuhiro for the Chinese translation.
El Terapagos Mexicano for the Spanish translation.