When well will a choa go into its cocoon on sonic adventure battle 2

whenever the riquirements are fulfilled. If your chao has a higher level than 15 in any techniqe (Power,Swimming,Flying,Etc) it will be close to evolving/going into its cacoon. And at that point all you really can do is wait for your chao to evolve. If you really want your chao to evolve than you can leave your system on all day, while you are in a chao garden (or all night, that's what i do) by the time you get back, your chao shall be evolved.

P.S. If you want to know a glitch for your chao, than do this..:While holding a animal or a machine part ( the glowing things) go close to your chao, not close enough to automaticaly give the animal to your chao but close so that when you drop it your chao will grab it and throw it back at you, and you can use it again! ( to do this glitch the best characters to use to do this will be sonic and shadow.)