Generally if your levels are high enough you can own them easy. I would pack the following:

Your choice x Lemonades (Lemonades are the cheapest ingame per HitPoint restored, and perfectly fill the HP that revives don't)
OR
Use Softboiled from Blissey (Can be used a lot, can restore the PP with ethers).

At least 10x Revives
Some Ether and Elixer, or Berries that restore PP. This is invaluable.

As for the team, you might need a competitive team.
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GET BLISSEY!

You know in Hearthome city, there is a hiker that gives you a Happiny egg? Don't find it useless. Get it to at least level 15, then level it up with the oval egg.
Fly to Veilstone, and start pumping this Chansey with about 10 Irons and 10 HP up. This should throw its happiness right up. So now, walk around really fast for a while, battle a lot with it. Also, give the chansey massages in the veilstone massage house place. Then level it up. It should evolve into Blissey - with the highest HP in the game. Those irons raised its pathetic defense. Start EV training it in defense, though.

Now, when someone on your team gets killed, or your team is especially hurt and damaged, switch to blissey. Heal the team while blissey takes a battering.

Perfect?
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You see throwing out a weak Pokemon doesn't work.
Anyway, here are all the types that the elite four uses:

Bug, Grass, Poison, Fire, Fighting, Normal, Steel, Ghost, Water, Ground, Rock, Psychic, Dark, Water.

Notice something? It's nearly every type in the game. Pack Pokemon with moves that can counter each type.
Infernape can use Grass Knot which is amazing on the Water/Ground types.

Other then that use the strongest Pokemon possible. If your pokemon are useful and are over level 60, you will win. Just pack the necessary items and a Blissey.

Palkia will own as its type allows it to learn Fire AND Water Moves. Spacial Rend is weaker then Roar of Time, but does not need a paused turn. Pack Earthquake if you have the TM or Earth Power for those weak-to-ground ones.