There are Four possible ways to get a shiny Latias/Latios or any Legendary.
1: Trade with a friend
2: Use Wonder Trade and hope to get lucky
3: Save before battling Latias/Latios and Soft Reset until you find one
(Soft Reset by pushing L,R,Select and Start at the same time)
4: Use the Pokemon Bank app to transfer shiny Pokemon from past games to ORAS
Hoped this helped! :)