Catch these Pokemon!
*Shiny Pokemon not promised
^^Dunno why I did that. xD
Start by ignoring the bush that requires Cut and just keep heading east, slipping through the gate. You'll see the Pokemon fly off to the west. Now go back and Cut the bush down, then head into the middle area of the ruins. Latios/Latias will fly off again, although this time over to the east.
Head over to the stairs to the east and climb them. Up there, Latias/Latios will fly by once more, this time to the west. Head west yourself, all the way over until you can move south. Down there, just shy of the item, you'll run into Latios/Latias once again. Pick up the Ultra Ball, since it's right there, then head all the way back to where the stairs were and keep following the path east until there's nowhere else to walk. After saving your game, step on the space at the very edge of the ruins and Latios/Latias will appear once again, although this time it will fight you!
I really couldn't be bothered putting this into my own words, so here it is.
You obviously have White 2, so Latias will appear. You can only fight Latios in Black 2, with the mentioned steps above.
Hope I helped :3