Sonic Adventure DX Director's Cut

Do you have Sonic Advance 1, 2, or Sonic Pinball Party? If you do, I highly suggest using the tiny chao garden in it to level up you chao. You will also need a Gameboy/Gamecube adapter. Just level up the one chao. It goes a lot faster with you just focusing on one chao. If you don't have any of the games I mentioned, I'm sure you can find them at a place that sells used games. As for the adapter, that may be harder to find.

I would recommend it having all the levels at 99. With the GBA to GC cable you are able to download a temporary choa garden that makes a copy of your choa. The garden is lost once you turn off you GBA, and if you bring the copied choa back to the GC, the copy replaces the original (though it still has the animal parts). And all Tiny Choa Gardens don't allow you to level up your choa though. But, I would recommend the GBA garden if you have a cartridge that has Tiny Choa Garden. Since you don't use Sonic Adventure Battle 2, it doesn't make your choa defunkitated. If you don't just give it choa fruit.