To get Latios or Latias SUPER easily, you'll need a strong L-40 or lower Wabbufet and a Pokemon that knows surf. Also you'll need some ultra balls and some super/max repels.
Surf to Mt. Pyre, go to the outskirts of it like the grass area of it. Look at your Pokedex for Latios or Latias and if its not there go to the second part of it where it gets foggy. Check it again.
If its not there, go back to the non-foggy area and keep doing it. Eventually, Latios or Latias will be in the area. It doesn't matter where you are on the outskirts because no matter what your in the Mt. Pyre area. So when it is in the area use a repel and search in the grass with WABUFFET IN FRONT.
When you get into battle, Latios or Latias cannot flee because of Wabbufet's Effect. Weaken it and catch it!