I think that Mega Altaria will be very good depending on what its EV spread will be. Here is a set I came up with:
Base Stats: 75/110/100/110/105/90
Altaria @ Altarite
Ability: Natural Cure
EVs: 112 HP 252 Atk 144 Spe
Adamant Nature
- Roost
- Double-Edge/Return
- Earthquake
- Dragon Dance
Moves
In my opinion, Mega Altaria will be best with a physical moveset. I used Pokemondb's type coverage calculator (
http://pokemondb.net/tools/type-coverage ) and found out that Fairy and Ground have the best coverage together, hitting almost every pokemon in OU for neutral damage, not including Skarmory and Talonflame/Charizard. This set is similar to Mega Charizard's bulky DD roost set in that it has Roost, DD, and 2 coverage moves. Since Mega Alt has such great typing in Fairy and Dragon, with key resistances/immunities to Fighting, Bug, Water, Fire, Electric, Grass, Dark, and Dragon, it can set up on a lot of pokemon roosting off damage, and DDancing. I chose Double-Edge over Return because Mega Alt will not have a very high attack stat, so I think the extra power will be necessary. However, if you don't want to take the recoil from Double-Edge, you can run return.
EV Spread
The given EVs in speed are to outspeed standard Timid Greninja after a Dragon Dance, so Mega Alt can OHKO Greninja with a Double Edge and not get hit hard with an ice beam. Here are the EVs needed to outspeed Greninja with different base speed stats:
85- 184 Spe
90- 144 Spe
95- 104 Spe
100- 64 Spe
(Basically you just want Altaria to hit 252 Speed)
I put max EVs in attack with an adamant nature because I doubt Mega Altaria will have an attack stat higher than 110, so it will need the Attacks to hit hard. However, I did not calc anything involving attack, so I don't know if it can use a little less.
I put the rest of the EVs into HP to provide for better bulk. If you wanted to, you could move the EVs into defense to take physical hits better, because I think that Mega Altaria will be taking more physical hits than special ones.
Teammates
As I said before, the only relevant pokemon that resist both or Mega Alt's coverage moves is Skarmory, Talonflame, and Charizard Y. Ferrothorn can also come in and take an earthquake, and OHKO Mega Alt with Gyro Ball. Because of this, I think that Scarfed Magnezone will be a really good partner with Mega Altaria. It can trap and kill Ferrothorn and Skarmory to free up Mega Altaria for a sweep. Heatran is also a good partner as it can deal with most sets of Talonflame and Charizard Y. It can also hit Ferrothorn and Skarmory with Lava Plume or Overheat. Mega Alt will appreciate a cleric like Clefairy, Sylveon, or Chansey because it won't like taking burns, paralysis, or toxic. Also, a defogger/spinner like Excadrill, Latios, or Latias will be good because Mega Alt will not like switching in when Stealth Rocks are up.
Conclusion
I would really like to hear what you guys think about Mega Altaria and its best possible sets, so just reply to this post with what you think.
Thanks for taking the time to read!