its a lot easier if you can trade the lati@s from another game as you can see where it currently is, but if not use repel in an area, if you dont encounter it run just out of the area and back in and try again
usually when your first slot pokemon level is lower than the pokemons in the area repels dont work(except if your level is higher) so I think that if you want to use an repel ,then get a pokemon with a level lower than lv.40 and use the repel.
max repels work best try going to a weak leveled area with another grassy route next to it as latios moves area when you do i always carry a wobuffet in front or a pokemon with mean look to stop it escaping good luck thats all i can give
This is the SECOND thread on Lati that you've made. There is a sticky devoted to asking for help with Latios and Latias, which I gave you in your last thread, here: