Incorrect - it's not just purifying all Pokemon, but you also need a japanese bonus disk - which were released as pre-order bonuses, and wouldn't be easy to find now, unfortunately, nor cheaply either, so I've heard. It says so on that link provided, but only if you hover your cursor over the tiny ellipes (...) behind it, hence you being mislead, methinks. =/ Silly bulbapedia.
Anyway, bulbapedia also holds the answer to your questions:
https://bulbapedia.bulbagarden.net/wiki/Pokémon_Colosseum#Ageto_Celebi
Level 10, with Confusion, Recover, Heal Bell and Safeguard. =)
I'll also add that instead of Celebi via the bonus disk, if you do purify all your Pokemon, you can get a Ho-Oh instead (level 70) - and there's no need for a bonus disk for that. =) That's done by starting and completing the Mt Battle challenge, 1-100 in Colosseum mode (i.e. not in the storymode) with a registered team after you've purifyed all 48 Shadow Pokemon. You can use Pokemon from the GBA games if you want - just be sure to register them, and from memory you fight Pokemon equal to your strongest Pokemon's level, minimum of level 50. Double battles, best out of four that you choose from 6 Pokemon, and it's pretty easy until the end which is not
that challenging IMHO (and you get continues too, methinks) - just timely.
Hope that helped. =)