Yveltal gives you a nice defogger and decent bulk. As well as sucker Punch for things like Shadow calyrex. And I also added taunt for things that want to set up or throw down rocks or spikes. And knock off is always good when you're expecting a switch and sucker Punch won't work.
The reason I want focus sash on mega Rayquaza was one guaranteed Dragon dance but you do have to be careful for things like triple axle..
After one Dragon dance he goes out speeds and one hit chaos scarf Kyogre or primal.