Hello, ISuckatmath! (Okay, that sounded odd...) So, seeing that I only defeated Elesa about 5 days ago, I can say I feel your pain. Anyway, though you might have allready gotten the answer you needed, I thought I would say MY personal strategy, in hopes it might help out some other lost trainer!
What I did to defeat Elesa was to first train my Pokemon. A lot. (Duh :D ) I don't know what everyone else does, but I ALWAYS want my lowest level Pokemon to be at least one level higher than my oponents highest level Pokemon. This strategy has, so far, worked wonderfully for me. In my head, I'm going to have to train them to that level eventually, anyways, so might as well do it early!
Okay, now that I've got THAT out of the way, I'll go on to my more... strategic strategy. My secret weapon to defeat Elesa was this: Blitzle. Most wild Blitzle (and Zebstrika) have a wonderful ability called "lightning rod" where electric attacks do absolutely no damage and boost the Blitzle's speed. This alone makes Elesa's signature move (volt switch) all but useless.
Now, I know what you're thinking: "But, since Blitzle's an electric Pokemon, all of its electric attacks will be really weak against Elesa's Pokemon!" Well, yeah, that is true. However, Blitzle learns an attack at level 18 called flame charge, which is a pretty useful attack at this stage in the game. Basically, its a fairly powerful fire-type attack. Are you thinking what I'm thinking? No? Yes? Maybe? Anyway, teach this WONDERFUL move to your young blitzle, train it up, evolve it, face Elesa, and bombard her stupid Emolga and Zebstrika with Flame Charge! This might not take care of all three of her Pokemon, but if you can train up a grass Pokemon (which electric attacks are weak on) then you should be A-OK!
I hope this helps! It worked for me!
P.S. The idea of just using one attack over and over again is generally not a good idea later in the game. However, in this case, I think it's an exception!