Getting Ho-oh isn't as hard as you've been led to believe. You can use either your Colosseum Pokemon or a GBA team. First thing to do is purify all 48 Shadow Pokemon. Either way, register the team (using a different held item for each), and select Mt Battle Challenge. Single or Double mode doesn't matter. Then just battle your way through the 100 battles, bearing in mind for each perfect victory you get a continue. Which is helpful, because they are not shy against using legendary and uber Pokemon. My Colosseum team usually consists of Espeon, Umbreon, Suicune, Raikou and Entei, plus a filler or supporting Pokemon (get your Vibrava to evolve and give it the Earthquake TM for a good suggestion). All the Pokemon you face are at the highest level of your own Pokemon, but at least L50 (so if your highest level Pokemon was 80, all the Pokemon you face will be L80, even if the rest of your team is not). Besides, it's really easy to level up in Colosseum, so level imbalance shouldn't be too much of an issue.