Of course, the team is built around Mega Rayquaza, and again, the premise of this team is to allow for riskier play of the God in order to punch holes in the opponent. In a meta where most of the mons are OU, the opponent is probably going to have to sac something every time Mega Rayquaza comes in, and with that in mind, the standard fully offensive set was chosen. Don't switch this mon in; send it in and make your opponent lose a mon every time you do so. These are its best 4 moves for general wallbreaking and destruction. If you wanted to use a more sweeping set, you might be better changing up the team a little.
A natural theme among teams is Healing Wish because the status of your God is so important. Generally it's strictly for survival purposes, but on this team, Healing Wish is mainly uses to let Mega Rayquaza severely damage or take out as many Pokemon as it can. The only Pokemon which is legal on a Mega Rayquaza team that learns Healing Wish is Latias, so that's why it's here. Draco and Psyshock are obvious, and I know you're looking at Thunder Wave. The reason it's there is because I didn't want Latias to both have to Defog and use Healing Wish, as keeping rocks off is so important for this team. Also, Xerneas is a huge threat to this team. Latias is one of the best Pokemon in this team for Xerneas to set up on, so we can't have that happen.
Now the team needs Defog somewhere and Skarm is, aside from the Latis, basically the best Defogger and provides good defensive synergy (well, as good of synergy as you're gonna get in this meta). I felt that rocks were better on something else, and I need all 4 of these moves anyway. I mentioned how there's a slight birdspam-esque core so it might be strange how I have Iron Head here, but it's nice to have another answer to Fairies, and plus, even though it's not common, Rock Polish Mega Diancie actually destroys this team otherwise.
So now we need rocks. Chomp is nice because with the three that are there so far it's easy to just Volt Switch around. I wanted an offensive set because (along with its nice speed tier and the fact that I have a Skarm already) aside from Latias, Garchomp is the only mon that Xerneas can really set up on. Offensive EQ puts it in Brave Bird + Espeed range whereas Tankchomp EQ actually does nothing.
The backbone of the team is complete, so now to make some nice additions to the offensive core. The two important things to notice are that Mega Rayquaza can sometimes spam Dragon Ascent, denting Flying resists and basically everything else, and that Rock-types are much more rare in this meta. With that in mind, Talonflame is a very natural addition. Although this team is catered to Mega Rayquaza annihilating everything it also turned out to be catered to a Talonflame sweep. The set is pretty clear, then.
Even though there's a lot of priority, having speed is nice. Again, this could've been a Scarf Thundurus-T or just a regular Thundurus or something but Kyub is nice because it's easier to spam Ice Beam in this meta, and Kyub can definitely clean up the game with Ice Beam or whatever, team dependent. You might be wondering why I'm talking about Ice Beam when this is clearly a physically-based set, and that's because it's really helpful to pressure Slowbro and Fairies even more. Also, a specially-based Scarf set is not really that good.