Skarmory @ Leftovers
Ability: Sturdy
EVs: 252 HP / 4 Def / 252 SDef
Impish Nature
- Toxic
- Roost
- Defog
- Whirlwind / Stealth Rock / Brave Bird
More or less the optimal moveset for Skarmory this gen. One cool perk is that Skarmory can come in repeatedly on Groudon without Fire moves to Defog away Stealth Rock, and it can reliably counter Extremekiller and most other SD Arceus (save Ghost w/ Shadow Force) which is great. Toxic and Roost are mandatory (Skarm really can't hurt anything without Toxic), Defog is probably why you're using Skarmory (so you can free up a moveslot on your Arceus), and Whirlwind is for phazing and can allow Skarm to emergency check Geomancy Xerneas if Sturdy is intact. However, WW isn't quite as useful on Skarm as it was last gen due to Spikestacking being a suboptimal strategy with Defog Arceus and/or Giratina-O on most teams, so Skarm can handle setting up Rocks if your team needs it. Brave Bird can also be used to punish M2X for attempting to Taunt and Amoonguss trying to nab free Regen healing. It is also your best defense against Mega Gengar unless you run Shed Shell. You can run Max Defense if you're terrified of Mega Kangaskhan and want to take hits from M2X better.
Pair with things that can take Fire/Electric moves and Kyogre. Palkia, Zekrom, your own Kyogre, Arceus-Rock/Water, and Giratina(-O) are good picks. Blissey is always a good Skarmory partner as well.