HA! I wish it was that easy for me to have gotten Ho-Oh...
The truth on getting Ho-Oh on Pokemon Colliseum
Purify all Shadow Pokemon (Once you get every single one from Story mode, including the Starters, Go to the Gas-Station like area and eventually you will fight somebody who looks like you who has the final Shadow Pokemon: Togetic purify it)
Beat the Battle Tower OUTSIDE of Adventure Mode (All 100 Floors)
Then you get Ho-Oh