Arcanine @Leftovers
Ability: Intimidate
EVs: 248 HP / 8 Atk / 252 Def
Impish Nature
-Flare Blitz
-Extreme Speed
-Morning Sun
-Will-O-Wisp
Tanks then 2HKOs with Flare Blitz
We're going to assume worst case for you (since only counter under worst case), but minus hax. So you always get 50/50's wrong, but he can't get double Protect (1/3 chance but whatever). Due to the nature of Flare Blitz as a recoil move, we'll be calculating both max and min rolls.
Toxic on the switch, Arcanine left at 360 (lefties -> poison).
Protect next turn, Arcanine left at 337 (lefties -> poison).
Seismic Toss next turn, plus Toxic will take you down to 191. If you Morning Sun, it goes off before this damage, and takes you to 237. ESpeed and Wisp are suboptimal, since the damage from neither will be able to help you KO faster, so we will only consider Flare Blitz and Morning Sun.
Assuming Morning Sun turn 3 (turn 1 being Toxic on the switch), you're at 237. Under our assumption, if you Flare Blitz, he
can Protect, and if you MS he
will SToss.
- If you Morning Sun, you will end with 214, leaving you in the same situation as last turn, but with less health than the turn before, and with more Toxic damage racking up. It's safe to say that Flare Blitz is better here.
- If you Flare Blitz (min 198, max 234 - recoil min 66, max 78), he can Protect this turn. Him Protecting will leave you with 168.
We're in the same situation as last turn, except we know that his Protect will fail (assumption). So your only choice is to Flare Blitz, since using Morning Sun will leave you in the same predicament as after turn 3, but with lower health (meaning he can Protect again). Recoil + SToss + Toxic will kill you for sure, so this does not work out.
Now what if instead of using Morning Sun on turn 3, we use Flare Blitz? You go to 191 after SToss + Lefties + Toxic, so if you Flare Blitz then your health will be between 125 and 137 (inclusive). If you Morning Sun the turn after, you get to 147 health, which is not worth since Toxic racks up 23 damage per turn, and 147 - 125 = 22, which would give you about the same time to live. We can conclude in this case that Morning Sun is also suboptimal.
After this, Registeel Protects, Arcanine goes down to a number between 56 and 68. It can be seen that at best, the next turn Arcanine will go down along with Registeel. If two min rolls are dealt, Registeel can survive due to the extra turn of Leftovers it gets from Protect.
I guess you technically "win" by taking out Registeel with Flare Blitz, since it does faint before you do. But I would not call this Arcanine set a counter to this Registeel. In reality, the Registeel player would have to make many correct predictions on when Arcanine recovers, but would also have a good chance of getting a double Protect. But I guess in reality, there are also other Pokémon that you can switch to.
Someone correct me if I'm wrong.
Also, here's a Doublade set to counter Registeel:
Doublade @ Eviolite
Ability: No Guard
EVs: 252 HP / 160 Atk / 96 SpD
Adamant Nature
- Iron Head
- Shadow Claw
- Shadow Sneak
- Swords Dance