So, your main team is going to look like:
Swampert, Aggron, Flygon, Absol, Charizard, and another Pokémon, right? The only glaring weaknesses you have at this point are Water and Fairy types, which can easily be countered with one of the Grass/Poison type Pokémon like Vileplume or Roserade. Roserade is much more favorable, as it gets better moves and has higher stats.
Also, do you plan on using all four Mega Evolutions? If you only intend to use one or two, I recommend taking advantage of Mega Charizard X or Mega Absol, since those are the two that could use the boost the most, really.
And, despite ORAS having a lot of HMs, you can compact it down to only two Pokémon for your HM users if you use Sharpedo (Surf, Waterfall, Dive, Rock Smash) and Tropius (Fly, Cut, Strength, either Rock Smash, sweet Scent, or whatever you want).