Do you have to play without saving until encountering it, and then soft reset if it's normal, and repeat until it's Shiny and save? Sheesh, that would take a while, and I already saved. XD
Soft reset is the best idea since the sheer luck of the fact latias/latios is even able to be shiny so yes you do have to soft reset and unless you plan on saving ALOT buy pokeballs (suitable for your time/location) and be ready for a fight.
source: experience and http://bulbapedia.bulbagarden.net/wiki/Shiny_pokemon to make sure latias is even obtainable shiny.
Just keep closing and opening your system until you encounter a shiny Latios. That is how my friend gets his shiny legendaries.
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