I Got my Rayquaza, by migrating it from my emerald game. Then I taught my Rayquaza four key moves. The first was surf, so it could take down fire, rock, ground, and more. The next move I taught it was Flamethrower, for those bug, steel, ice, and grass pokemon. The third move I taught it was Crunch, for ghost psychic pokemon. The last move that I taught it was Thunder, for the high flying pokemon ,as well as water pokemon. Then I trained my rayquaza to level 100. Now it needed and item so I gave it a leftovers. Then I healed it and challenged the elite-4 and champion. I breezed through the elite-4. when it came time for the champion I was able to beat her. By using a combination of all four of my moves. That’s how I was able to beat the elite-4 with just my Rayquaza.
I used Rayquaza but you could use anything with four differnt types of moves
*note you can use any pokemon with Any combination of moves You can have fire,flying,dark,water,or fire,flying,grass,fighting
it doesn't matter what you use but use strong moves and preferably something strong against bug,fire,ground, and Psychic type pokemon