Its not possible in just ORAS on its own. There are around 13 Sapphire exclusive pokemon (including the legendaries) and plenty of none ORAS pokemon (Serebii have a full list if you google pokemon not obtainable in ORAS)
My biggest tip is too put up random starters for pokemon you want, search the GTS alot and wonder trade as much as you can. I recently completed my dex in Ruby and X and i'd say about a quarter came from wondertrade, you get some real gems (ignoring the zigazoons)