How do you get a chao with 9999 stats on Sonic Adventure 2 Battle

To make the best chao possible you would first need to evolve it through many lifetimes, leveling up a stat for each lifetime until all stats are S. ( When your chao evolves one of it's stat ranks increases. For example if your chao's run ability has a rank of B it will have rank A when it emerges from the cocoon)
When you have an all S chao you could make it a chaos chao to get as much time as you want to increase it's stats, as a chaos chao doesn't die with age. Making it a chaos chao is probably not mandatory though. The stats of a chao with an S rank on all stats grow faster, so this is why the ranks need to be increased.

When you have a chaos chao with all S ranks all you need to do is keep giving it tubes and animals until all of it's abilities are level 99. I don't know if they will reach 9999 but that would be the best chao possible.