Well, if you want to handle a few at a time (growing), I'd say dust off the ol' EXP. Share, or put your two stronger in the daycare and personally train the weakest by throwing yourself at the Elite Four repeatedly. That's how I raised my Salamence to level 76. After that I chased Trainers, then went back to the E4. Remember, in Open Level all the frontier opponents are the same level as your highest, so keep them close together.
If you still have Steven's Beldum- Well, a Metagross is pretty powerful (They can learn a lot of different special and normal moves, as well as Meteor Mash at level 55? or something, which is a great move) and if you take the time to train the Mudkip as well, having a Swampert in the 80+ level range (try to raise your other Pokemon too) and a Metagross anywhere from level 60-90 is really powerful.
With the Corsola thing, you should teach it typical Corsola good moves- Protect, Recover(you already have it XDD), and Spikes can help give you an advantage, and if you saved your Water Pulse TM, it's really good when used by a Corsola. Keeping Earthquake might be nice, but most trainers have a decent earthquake avoider these days. Get rid of Surf- it's not the greatest move unless it's against a disadvantaged or weaker opponent and it only gives you an edge in double battles, really. Ice Beam is nice, ice moves are generally quite useful when you're fighting most teams.
All in all, you should really aim for whatever you can handle. Memorize the type chart or repeat the "Fire Grass Water Fire Grass Water" cycle to yourself every night. Get some nice powerful TMs, and try out EVERY new move against the elite four. Some aren't as strong as ya think. Get a few 1-hit K.O. moves, and raise the levels as high as you can. Play the Battle Factory (rental pokemon) so you get used to your types and the feel of using different pokemon. Use the Battle Dome and Tower to practice pure strength, the Pike to test "stress" (psn, sleep, frz, paralization etc.) conditions- can you hold your head even if your team is all paralyzed, or are you willing to wait until ______ defrosts?
Battle Pyramid is a good time-killer, if you just want an assortment of mindless battles and strong "wild pokemon" encounters.
To test the attacks themselves, go to Battle Palace- you need to be able to trust your Pokemon and their moves enough to rely on any move being used.
