Before challenging the elite 4 stock up on revives and hyper potions.
You didn't specify a team or not, so I would say catch rayquaza at the sky pillar. It will be level 70 and you need the mach bike to get to the top of it. rayquaza will know outrage, rest, fly and extremespeed.
You can give it any held item you want.
You can OHKO everything on Sidney's team with outrage.
Phoebe can be beaten by spamming outrage as well, but watch out for the second dusclops because it knows ice beam. Also make sure the first dusclops doesn't stall out all of outrage's pp with protect.
Glacia is harder because she has ice moves, but she also likes to set up hail early even though rayquaza's ability negates it. You can beat her by teaching rayquaza the thunderbolt TM obtained by doing the new Mauville side quest, because 3/5 of her team is weak to electric.
If you somehow can't beat the glalies by using outrage then teach it the overheat TM that Flannery gives you or fire blast from the Lilycove department store.
Drake is easily beaten by spamming outrage of course because the whole team is weak to dragon.
Steven can be beaten by teaching rayquaza surf because half of his team is weak to it. He will lead with skarmory which dies to thunderbolt, and metagross can be killed with overheat. Cradily can be beaten by a flying move or outrage, preferably fly because that hit's on cradily's weaker defence. If you still can't beat Steven with rayquaza alone then the rest of your team must be a high level by now and can deal with him.
Also make sure to fully heal/revive rayquaza between each elite four member and the champion.
Hope this helps if you haven't beaten them already.