(everything bolded is either important, noteworthy or my personal preference)
Flygon @
BrightPowder / Leftovers
Ability: Levitate
EVs: 44 HP / 152 Atk / 252 Def / 60 Spe
Adamant Nature
- Earthquake
- Roost
- Substitute
- U-Turn
This Flygon is specifically designed to have more than a
100% chance to win against that specific Primal Groudon set. It outspeeds him by 1 point, which is a huge advantage, because if Groudon wants to outspeed, he has to waste an entire turn to do so. This Flygon also has this
100% chance to 2HKO, which means in a bad situation (which only happens against TAS) he can quickly outspeed and get the kill with only 2 hits. Because of Levitate, we only need to focus about Groudon's Stone Edge, which has its perks, like only having 8 PP, and only having a 80% chance to hit. Relevant calc's:
252+ Atk Groudon Stone Edge vs. 0 HP / 196 Def Flygon on a
critical hit: 110-130 (
36.5 - 43.1%)
+2 252+ Atk Groudon Stone Edge vs. 0 HP / 196 Def Flygon: 147-173 (
48.8 - 57.4%)
+
2 252+ Atk Groudon Stone Edge vs. 0 HP / 196 Def Flygon on a
critical hit: 221-260 (
73.4 - 86.3%)
152+ Atk Flygon Earthquake vs. 120 HP / 0 Def Groudon: 186-218 (
50.1 - 58.7%) --
guaranteed 2HKO
Stealth Rock is the only entry hazard that can make a difference here, but that difference is pretty much unnoticeable. Another great thing about this set is that it can check the Groudon multiple times every match. He will either force it out, or kill it. He can use U-Turn for momentum, which also has great utility in general. But the most noteworthy thing about this thing, is that he can check/counter Groudon both offensively (like previously explained) and defensively (in a way). That is because of the rather unique combination of Substitute, Roost and BrightPowder (and his Typing + Ability, ofc). The reduced accuracy caused by his item makes Groudon's Stone Edge a shitty move with only a 72% chance of hitting, which can be abused to its fullest with sub and Roost. Sub can also just be used to stall out the PP of Stone Edge and it also works well when if he retreats.
The 4th move (and to some extent the 3rd one) can be swapped out with a variety of other moves. While the one listed above was picked to fare the best against that Groudon, other moves can be used, and only makes a really small change overall. Defog, Toxic, Double Team, Tailwind and even Sand Tomb (to trap either Groudon or a switch-in) can be used and still get some satisfying results.
There is another EV spread that also works on this poke, which gives more defense, but reduces the chance to 2HKO Groudon to "only" 95.25%. The bulk you gain is more than the loss of attack, but it reduces the chance to win against that Groudon to "only" around 99.9%. That EV spread is: EVs: 252 Atk / 196 Def / 60 Spe with Impish Nature. Leftovers can also be used instead of BrightPowder obviously. It is more viable in general, but inferior in this specific case.
If the that Groudon were to carry another coverage move, like Dragon Claw, Then Flygon is pretty much dead. He can live one hit, but not 2 hits, a crit or one +2 hit. It can miss, tho. With a Haban Berry equipped, he will beat the Groudon if he sets up Swords Dance and does not crit on the following turn. 2 regular Dragon Claws will Kill regardless