If you have completed the main game, and if you have both Tornadus and Thundurus (you will have to trade one from the other game) then you can catch Landorus at the Abundant Shrine.

How to Reach Abundant Shrine

To reach Abundant Shrine, go to Route 14, go up the waterfalls, then use Surf to make your way to the west. Go into the forest entrance to reach Abundant Shrine. To make Landorus appear, walk up to the shrine in the northern part of the area and check on it while Thundurus and Tornadus are in your party. Landorus will float down and attack.

Tips for Catching Landorus

If 30 turns pass, the best pokéball to catch Landorus is the Timer Ball. Otherwise, if the battle started after 8PM and before 3:59PM, you can use a Dusk Ball. If you don't have that, try using Ultra Balls.