I hated that gym in emerald...But maybe that's because I tried to train my team evenly on that file. Helluva mistake that was. Aron was my only grounder too.
First, you should see if you can teach it any more ground moves, like rock tomb. Rock tomb should help when you're against that pesky Magneton. If you want a stronger ground move for your Aron, I suggest you look in the game corner place for one....So let's see..., a nice take down (here's hoping your Aron has rock head) should KO the Electrike, if you aren't paralyzed by it that is. Magneton should either get KO'd by rock tomb, or self-destruct itself because it's an emo.

Manectric's the toughie here, so you should just use your best ground move and hope you don't die. Treeko/Grovyle should be good backup at this stage of the battle...I forgot Voltorb...Well, a good take down or ground move can kill him anyway. After you do the above steps but before you battle Wattson, train like your life depended on it. And it does. *evil grin*
Edit: Just for the record, I know Rock tomb's a rock-type move. I'm saying to use it against Magneton because Magneton's a
STEEL-TYPE! Just sayin'