Though this calc:252+ SpA Choice Specs Volcanion Steam Eruption vs. 252 HP / 40 SpD Suicune: 126-149 (31.1 - 36.8%) -- guaranteed 4HKO after Leftovers recovery
may seem appealing, volcanion is not a counter or switch-in to any sort of suicune at all. The most it does is sit there for 8 turns until earth power pp gets drained off and you're forced to struggle to death.
And if you're switching into a calm mind, then good luck breaking this thing's sub.
252+ SpA Choice Specs Volcanion Steam Eruption vs. +1 252 HP / 40 SpD Suicune: 84-99 (20.7 - 24.5%) -- possible 6HKO after Leftovers recovery
252+ SpA Choice Specs Volcanion Sludge Bomb vs. +1 252 HP / 40 SpD Suicune: 92-109 (22.7 - 26.9%) -- possible 5HKO after Leftovers recovery You're insanely lucky if you manage to break past this and poison it.
This also includes any non-choiced sets that comes in, but those mons are just gonna take longer to pp stall. Heck I might even say that Volcanion loses against sub cm in a 1v1 anyways cause you're technically only at 4 pp with super scald, which suicune can stall out via sub and protect. And keep in mind if you arent specs, you arent breaking any subs.
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With that out of the way, I'm gonna suggest something that might be surprising. Choice Band Victini
This mon is one of the strongest wall breakers in the tier, being one of the only pokes that ohko's clefable without any prior damage.
252+ Atk Choice Band Victini Bolt Strike vs. 252 HP / 0 Def Suicune: 344-406 (85.1 - 100.4%) -- 6.3% chance to OHKO
252+ Atk Choice Band Victini Bolt Strike vs. 252 HP / 0 Def Suicune: 344-406 (85.1 - 100.4%) -- 43.8% chance to OHKO after Stealth Rock (and protect)
252+ Atk Choice Band Victini Bolt Strike vs. 48 HP / 0 Def Tyranitar: 179-211 (50.7 - 59.7%) -- guaranteed 2HKO
0 SpA Suicune Scald vs. 0 HP / 4 SpD Victini: 158-188 (46.3 - 55.1%) -- 63.3% chance to 2HKO
+1 0 SpA Suicune Scald vs. 0 HP / 4 SpD Victini: 236-278 (69.2 - 81.5%) -- guaranteed 2HKO
Though at a glance, it doesn't look as if it's possible for tini to switch in and beat Suicune, but it's possible. The worst outcome is tricking a band onto suicune, and lets be honest. Suicune's utility is pretty much gone at that point anyways.
There are several scenarios that might take place, though victini exits with a fainted or crippled Suicune in any scenario
The first, and the worst situation is when suicune scalds on the switch, possibly anticipating that tini is your answer. Here, you will have to go for trick. After the trick, suicune has no boosts and can be easily revenge killed by anything that's not weak to scald. Additionally, you might want to go for bolt strike hoping for a crit, parahax, or high roll, or weakening suicune to very low health if you have no way of revenge killing a banded suicune.
The second, is when suicune cms on the switch, maybe anticipating a switch into a special attacker like latios or gengar. Here either move will work, but because suicune is at +1 already, it will generally be easier to deal with suicune if he's hit by bolt strike instead of trick. There's also a 50% chance that you'll get ohko'd after stealth rock, excluding crits, so you'll have to play your luck with bolt strike and see if you get any lucky rolls.
The third, and the best situation is when suicune protects or subs on the switch, wanting to heal up from SR damage or anticipating a toxic or will-o-wisp from hippo or rotom-w. After taking sub damage Bolt strike will usually KO after several rounds of protect. If you happen to switch into protect (or pivot out via u-turn, baton pass, or volt switch), know that tini should win if you keep spamming bolt strikes unless your opponent gets lucky with protects.
Disclaimer:This mon, along with a revenge killer, is usually enough to deal with vincune, but you have to be aware that the opponent might switch out at any time, so this mon is a shaky check at best.
