There is away to help catch them. Move back and forth between routes until the Pokemon shows up on the Pokedex Location as the same route as you. Then, put a fast Pokemon that knows Mean Look or Spider Web (I always choose Crobat) first in your team (making sure it is the same level or lower than the legendary.) Save your game (just in case.) Use a Max Repel before going into the grass. Then, go into the grass. You probably know what to do then. The Max Repel is so that you don't have to deal with other wild Pokemon.