Ho-oh is
fairly easy to find compared to Lugia. Go back to Ecruteak City and go into the
northern house. Go in all the stairs and you'll eventually come to a large
tower. Go inside Tin Tower, and go to the top after the winding mazes and
teleport pads and you'll find Ho-oh. You better save your game before fighting
Ho-oh, because like Mewtwo, there is only one. Try getting Ho-oh down to as
little HP as possible, and put it to sleep. Ultra Balls should do the trick.