not op at all...
If it gets a swords dance up and is holding a life orb it can kill pretty much every Pokemon in the game in one or two hits. Only 37 Pokemon (if you count Deoxys's forms seperately) can outspeed Mega Ray (without the use of something like a choice scarf) and Dragon Ascent is just as powerful as Brave Bird only it trades recoil damage for a defense debuff that most Pokemon won't last long enough to get a chance to take advantage of.
Multiscale Lugia is widely considered in the competitive community to be the greatest defensive Pokemon in the game so keep that in mind when I show you this:
+2 252+ Atk Life Orb Rayquaza Dragon Ascent vs. 248 HP / 252+ Def Multiscale Lugia: 214-252 (51.5 - 60.7%) -- 94.1% chance to 2HKO after Leftovers recovery
You could try to use priority moves like Ice Shard or Bullet Punch to try to get the jump on Mega Rayquaza except Mega Ray can counter that by using Extreme Speed (which outprioritizes everything) if the person playing it can see it coming.
So how do you deal with a Pokemon that can't be walled and can't really be outsped? You can't really. No Pokemon can stand up to that kind of power on it's own.
Maybe your hackmons can stand up to it but actual legit Pokemon have a very hard time dealing with Mega Ray. With my current team I'd have to either sacrifice one of my Pokemon or be lucky enough to have the right guy on the field to get rid of Mega Rayquaza. A team that is not as prepared as I am would more then likely be wiped off the map completely.
Oh and your sturdy shedinja trick really isn't as clever as you think it is, FYI.