I don't envy you. Training for Sp.A in Ruby, Sapphire, and Emerald is notoriously difficult. Your best bet is Spinda, which I bet you were already using. TBH, if Phoebe is what this is for, I don't think EV training is necessary. If you just focus on leveling up some OP Emerald dragon/steel/psychic and don't worry about EVs then you shouldn't have a problem.
Remember your advantage! This isn't competitive, you're the king of full restores and max revives!
In case you were planning to actually use this Mightyena for competitive at some point, I went ahead and looked the probability of finding a Spinda on Route 113: 70%. That's some helpful motivation courtesy of Smogon (http://www.smogon.com/ingame/guides/emerald_ev_manual )