Your set for blastoise is pretty good, I forgot it could learn dragon tail which is pretty useful on blastoise. Surf and water pulse have the same power in mega so it's a choice between convenience for surf and confusion chance for water pulse. The other attacks are just perfect for mega blastoise
For talonflame you may want to use flare blitz instead of flame charge since flame charge is pretty weak and talonflame doesn't want to receive a counter attack. It can relearn flare blitz at the move relearner and heart scales are easy to find. I used acrobatics instead of steel wing for a powerful drawback free attack but I can see you might want steel wing to make talonflame your fairy slayer. That's your call but remember outside of dealing good damage to fairies steel wing is pretty useless on talonflame.
For Chesnaught you can replace brick break with hammer arm if you can find another heart scale or get to level 60 for more power, he's already so slow the speed drop won't amount to much. If not then brick break can work in the mean time. I sure hope poison jab is there to slay other grass types it won't be much use against fairies who are all faster and will blast chesnaught away before he can move. Though if you are using it to cover grass types you already have talonflame who can do it just as well if not better. Chesnaught has many options so you can run practically anything on him. Personally I'd run spiky shield or stone edge but it's your call.
Meowstic and Tyrantrum are spot on so you can leave them. You could try rock polish for tyrantrum and it can work if you can find space for it.
For Xerneas if you are running the story normally you probably won't see close combat since it comes at level 80. Use something like thunderbolt in the meantime.
While you are running through the game keep in mind what moves are available at the particular moment. Rock tomb is great on tyrunt until you can find something better, aerial ace is good on talonflame till you can get a better stab and so on.