Last night I was horde exp farming in Victory Road on X, trying to get my national pokedex filled up, when I spot a shiny Geodude.
Using a Vileplume (for Sweet Scent and Petal Blizzard for easy and quick farming) as my only attacker, I couldn't really use it for taking out the other Geodudes to capture the shiny. So I switched out to the next highest level poke (happened to be a Shelgon) to take the Geodudes out one at a time.
Geodude Nonshiny #2 used Magnitude 7, took out half of the shiny's HP.
Geodude Nonshiny #4 used Magnitude 7, took out the rest of the shiny's HP.