I just want to keep this here if you just want a written out copy.
As you enter the dreamyard, (after when you beat the game) Latios will show up and growl at you. It'll run off after it does so. Well, you're going to have to chase it down in order to get Latios. I suggest carrying some Dusk, Timer and Ultra Balls to improve your chance of getting Latios.
Forget about the bush that requires cut, and just keep heading east, slipping through the gate. You'll probably see the Pokémon fly off to the west. Now go back and cut the bush down, then head to the middle area of the ruins. Latios will fly off again, this time to the east.
Head over to stairs to the east and go up them. Up there, Latios will fly by once more, this time to the west. Head west, all the way over until you can move south. Down there, you will run into Latios again. Pick up the Ultra Ball (its right there, why not?) then head back all the way to where the stairs were and keep following the path east until there's nowhere else to walk. I suggest saving your game and then step on the space at the very edge of the ruins and Latios will appear once again, this time, wanting to fight you!
Just here as an written copy. For all those who like to read C: