I found a way to pretty much beat the elite four with only a few pokemon so here is what you need to do.

Go to challenger's cave and catch a Rilou and go to Giant Chasm to catch a Ditto. Or you can catch a female rilou and a compatible male. Evolve the Rilou into Lucarrio and breed them (the Diito, or the compatible male) (This way you get more EV's) Once you get eggs try for a nature that raises SP. Atk. I used Rash on mine. Then in the battle Subway go get a Power Lens and put it on your newly hatched Rilou. Go to Celestial Tower and go to the Floor with the nurse. Go battle the Litwicks and/or the Elgyems. For each one they give you 1 Sp Atk EV, plus the Four from the Power Lens.

Be sure your Rilou learns Nasty Plot. That will boast the already High Sp. Atk. Then once it Evolves be sure it also learns Aura Sphere, Dark Pulse, and Psychic. Aura Sphere will take care of the Dark user, Psychic will take care of the Fighting user, and Dark Pulse will take care of the Ghost and Psychic users.

Train it up a lot. Preferably to the High 70's and 80's. By then you should have maxed out Sp. Atk. When mine was around those he had a Sp. Atk of 270, and attack of 200. Around lv 90 he/she should have around 300 ( mine had 302 at 90). Take your best team and this Lucarrio, and a lot of Full Restores, and Revives( incase Lucario Faints.) and take them on.

On my second time around the Elite Four I used mostly this Lucairo,

then I had a Reshiram lv 84 with the moveset of Dragon Claw, Fly, Crunch, and Fusion Flare)

and a Lugia I transfered over from Soul Silver at lv 85 with the move set Recover, Charge Beam (to boast its sp. atk once in a while) Aeroblast, and Psychic.

I know not everyone can get a Lugia if you cant dont worry! Lucarrio and Reshiram should do it for you, It worked fine for me. Lugia was just helpful for the fighting user.