Teach a Clefairy Double Edge from the move tutor for it either in FR/LG (in Victory Road) or Emerald (in the Sootopolis Pokecenter). Then, trade the Clefairy over to D/P and evolve it with a Moon Stone and there's a chance that it's ability will switch to Magic Guard.
Of course, there's no guarantee of that working, so the easiest thing to do would be to make a clone of the Clefairy in Emerald using the cloning trick, then send the clone over to D/P and evolve it there; if the clone has Magic Guard, then teach the real Clefairy in Emerald Magic Double Edge and trade it over. If not, then get a new Clefairy and start the process over again until you get one that does have Magic Guard.