Colosseum was a fucking abomination. Shitty plot, shitty models, shitty execution all around. Genious Sonarity is supposed to be composed of former members of all these great studios, like Enix and shit, and the game they put out was something that looked, felt and played like it was thrown together by a bunch of interns.
The sequel, Gale of Darkness, wasn't much better, but at least they made the whole "purification" thing a lot easier. Convluted, but easier.
I still haven't finished Colosseum to this very day, because of the total bullshit that that game is. Besides, you can get all the GSC starters and legendaries from Fire and Leaf on the GBA.