Since there is no way to increase Shiny Odds in Gen 3, the odds will always be 1/8192. However, there are a few methods of finding Shiny wild Pokémon. There are 2 methods that are really the option for getting a shiny Ralts. However, I believe that Soft Resetting/Running Away will take way too long considering the odds of finding a Ralts on Route 102. So, I believe that breeding is your best option here. Here is a basic overview:
Catch a Ralts on Route 102 (Preferably Female, but Male will also work). Next, you go to the Daycare Center and breed it with a Male from the Amorphous Egg Group (If Female), like Koffing or Duskull. Now, put a Pokemon with the Ability of Magma Armor or Flame Body (like Slugma) in your party and start receiving those Eggs. If your Ralts is Male, breed it with Ditto instead. The odds will be 1/8196 and cost you Pokedollars, but I still believe that it is the most efficient method due to the odds of finding a Ralts in the Wild.
Note: There is an extremely complex method of manipulating RNG in Emerald for finding what you want, but I prefer simplicity, so I still recommend breeding for your Ralts.
2nd Note: You could have an Illuminate Ability Pokémon at the front of your party to increase Wild Pokémon Spawn chances in order to do the Run Away/Soft Reset Method. If you want to use this method to save some Pokédollars, go ahead.
Source: https://www.reddit.com/r/ShinyPokemon/comments/54upbx/fastest_way_to_get_raltskirliagardevoir_shiny_in/
Emerald RNG Manipulation Guide: https://www.smogon.com/ingame/rng/emerald_rng_intro
Hope this helps!
