Shaymin-Sky is a sleeper inherently incompetitive threat. I'm glad that our esteemed Uber Leader (~) Sweep managed to identify and test the threat, before it got out of hand just like Xerneas, Salamence-Mega, Kyogre-Primal, Groudon-Primal, Gothitelle, and Gengar-Mega did. I want to bring a completely pertinent and relevant calculations to the attention for the community to consider the severity of the threat of this Pokemon.
+2 252 Atk Life Orb Arceus Extreme Speed vs. 0 HP / 0 Def Chilan Berry Shaymin-S: 203-240 (59.5 - 70.3%) -- guaranteed 2HKO
252 SpA Shaymin-S Seed Flare vs. 248 HP / 252+ SpD Gothitelle: 126-148 (36.7 - 43.1%) -- 99.1% chance to 3HKO after Leftovers recovery
These calcs demonstrates Skymin's incredible offensive and defensive prowess. The fact that a boosted Ekiller cannot immediately faints a Chilan Skymin risks a Healing Wish or being Revenge Killed if it's weakened sufficiently beforehand. Skymin is one of many pokemon that can muscle through and 3hko Gothitelle and I encourage you all to use it against Melee Mewtwo for ORAS, XY, and BW games.
However, as incredible Skymin is, it does possess a crippling weakness to our favorite C- Ranked Pokemon:
252 SpA Life Orb Turboblaze Reshiram Blue Flare vs. 0 HP / 4 SpD Occa Berry Shaymin-S: 386-456 (113.1 - 133.7%) -- guaranteed OHKO
I encourage ladderers to start using Flame Charge LO Reshiram to counter Shaymin-S. However, be careful, since Air Slash can muscle through Reshiram occasionally. But do not worry, the luck is on your side unless you're playing Steelphoenix!