You have under-leveled Pokemon!
Ok. Champion Iris is usually easy to beat, IF your Pokemon are around the same level as hers.
Basically all you need to do is level up your Pokemon. Victory Road is the right place to do that.
After your Pokemon are at least lv 50 you can go ahead and try it again.
As for her team, I think Hydreigon is the main threat.
Charge Beam (Electric)
Dragon Pulse (Dragon)
So you can't go again's it with Emboar or Beartic. I would usually go for a single fighting type Pokemon, but since you don't have that, I think Ampharos is your best bet here. It's the only Pokemon Hydreigon isn't super effective against in your team.
Her other Pokemon shouldn't be a problem.
EDIT: You also have a very unbalanced team. You have 2 flying type Pokemon + Flygon which can learn Fly. There isn't much you can do about it at this point of the game, but keep in mind that a good team consists in having different types of Pokemon.