I believe this works for Sapphire, since it did for D/P/Pl for catching Mesprit. You have to be at the edge of the route though and must have Latias data in your game.\
Just run to the other route meaning walk 1 step into the Route and Latias will be at a different location, keep doing this until Latias is on the route you are on, use a Repel and go through the Grass. Also remember, the Pokedex helps a lot ;].