In order to gain lots of experience at once, you must focus on raising one pokemon at a time, (No Exp. Share) and battling against Pokemon of a higher level. There is no easy way to do this other than finding a high-level Pokemon (At least at Level or above) with plenty of weaknesses and repeatedly battling them.Trainer battles result in much more Experience than wild pokemon, and rarer pokemon give more experience than common ones.
Chat with our AI personalities
you evolve a duskull at lvl 37 one of the best ghost Pokemon on emerald
at level 20
I've looked on the internet and so far the easiest Pokemon to train is Happiny, Chansey and Blissey, I'm not sure if you can find them on ruby, sapphire or emerald, but they only require 800,000 exp to lvl 100!!! Bug Pokemon. -Vicky is Sicky.
You should get a Duskull and evolve it [it evolves at lvl 37 to Dusklops] then get a Shuppet the evolve that [it evolves at lvl 37 too] then teach it a ghost type move or dark type move, then train and that is how to beat the 7th gym leader in Pokemon Emerald. Good luck!!
You get Gardevoir by evolving Ralts to first Kirlia at lvl 20 and it will evolve to Gardevoir at lvl 30