Defeating rayquaza postgame help

Title. Dragon ascent is hilariously broken and one-shots my entire near level 60 team. Seems like the programmers pulled a practical joke here; climb 34 floors of easy dungeon and lose immediately. How do you beat Rayquaza? It seems impossible. I even used orbs to power up and one-shot city still. I want to progress to where my Pokémon evolve; I need those stones but none of the dungeons that drop them are unlocked yet so I am trying to get past the postgame bosses.