I am newer to the tier but ladder pretty decently so here are my views on the suspects as I have been saying in the RU chat a lot and why.
First we have Serperior. Game freak bestowed us with an ability that took this mon from garbage to gold in one fell swoop. My biggest issues with it are A it has a base speed of 113 leaving only: Ambipom(included for accuracy but NEVER use it), Cincino, Dugtrio(this is going to get you super far for sure), Mega-Pidgeot(Lets run a suspect to counter a suspect=healthiest meta possible am-i-right?), Jolteon, Accelgor(seems to be rising in use), and Mega Sceptile able to outspeed it. Now my second problem B is that it has a stabbed 130 power Nasty Plot. This is not in any way comparable to Malamar's underwhelming 120 power attack that gives him an attack and defense boost; this is a legit threat. While A LOT checks Serperior(Going to list every RU mon in alphabetical order: Amoongus, Braviary(especially if scarf), Cincino(252 Atk Choice Band Cinccino Tail Slap (5 hits) vs. 0 HP / 0 Def Serperior: 260-315 (89.3 - 108.2%) -- approx. 12.5% chance to OHKO and 252 Atk Choice Band Cinccino U-turn vs. 0 HP / 0 Def Serperior: 192-228 (65.9 - 78.3%) -- guaranteed 2HKO), Cobalion, Delphox, Dragalgae, Drapion, Eelektross, Emboar, Escavalier, Fletchinder, Golbat, Houndoom, Magneton, Moltres, Pidgeot-Mega, Registeel, Shiftry, Skuntank, Virizion, and Whimsicott not very much counters Serperior.
From what I have seen the list of Counters for Serperior is short and most are shaky. The (mostly) agreed upon ones are Amoongus, Bouffalant, Fletchinder, Golbat, and Moltres. Now I am going to break down how effective each is and why it is or isn't shaky imo as most of these are. (All of these calculations will be using the 3 attacks set of Leaf Storm, Hidden Power Fire, and Dragon Pulse with 252 special attack, 252 speed, timid nature holding a life orb unless stated otherwise)
Golbat, Registeel, Skuntank, Bronzong, Amoongus come across as pretty solid counters. There are others including Sliggoo and Togetic that pretty much laugh in Serp's face.
Amoongus: I used to be told standard Amoongus was 252 hp, 160 def, 96 spec def bold. That seems to be changing now so will use this set first. To counter the snake you must swap into the attack. So off the bat you take 252 SpA Life Orb Serperior Leaf Storm vs. 252 HP / 96 SpD Amoonguss: 51-60 (11.8 - 13.8%). Now it outspeeds you and is at plus 2(assuming you didn't miss) and fires off +2 252 SpA Life Orb Serperior Hidden Power Fire vs. 252 HP / 96 SpD Amoonguss: 255-302 (59 - 69.9%) while 0 SpA Amoonguss Sludge Bomb vs. 0 HP / 0 SpD Serperior: 176-210 (60.4 - 72.1%). Amoongus either retaliates with 0 SpA Amoonguss Sludge Bomb vs. 0 HP / 0 SpD Serperior: 176-210 (60.4 - 72.1%) which fails to kill and Serperior gets one more hit off potentially killing Amoongus if not itself depending on the roll(we have lost 10% from Leaf Storm, 10% from HP Fire and any further attack is another 10% so a high roll makes the next hit a double down) or Amoongus 0 SpA Amoonguss Clear Smog vs. 0 HP / 0 SpD Serperior: 102-120 (35 - 41.2%) which does pathetic damage but resets the snake. If we follow the latter however we now have taken 59%-69.9% as black sludge has made up for the first Leaf Storm hit. Now Snake outspeeds and can 252 SpA Life Orb Serperior Hidden Power Fire vs. 252 HP / 96 SpD Amoonguss: 130-153 (30 - 35.4%) which means a high roll on the plus 2 hidden power fire can kill Amoongus so what a counter?! This isn't even factoring rocks which if are up 59%+11.8%+30%=100.8 and rocks have canceled out the sludge recovery. That's 3 absolute min rolls still equaling a kill. Not exactly a counter.
Now that set is practically dated due to Serperior so lets try a max HP, max SPec def spread calm.
252 SpA Life Orb Serperior Hidden Power Fire vs. 252 HP / 252+ SpD Amoonguss: 101-120 (23.3 - 27.7%) -- possible 5HKO after Black Sludge recovery
+2 252 SpA Life Orb Serperior Hidden Power Fire vs. 252 HP / 252+ SpD Amoonguss: 198-234 (45.8 - 54.1%) -- 3.9% chance to 2HKO after Black Sludge recovery
So this spread or something similar(have not yet been informed of the agreed upon spec def Amoongus spread) can handle Serperior easily but if we are breaking long established sets to handle one mon doesn't that sound a bit like overcentralizing? Also Amoongus as defensive checks a lot of threats if not counters them and now it cant do that so the role Amoongus once filled has been swallowed up.
Metagames characteristically adapt and change as new threats are introduced. This entire argument seems to be more about an unwillingness to change, rather than proof of overcentralization. Is specially defensive Amoongus only really useful for handling Serperior? While Amoongus itself is not going to handle certain other threats, is that loss also significant?
Bouffalant is a solid counter to Serperior. Immunity to the Leaf Storm means it cant set up but for completeness' sake a few calcs
252 SpA Life Orb Serperior Dragon Pulse vs. 172 HP / 0 SpD Bouffalant: 88-104 (23.5 - 27.8%)
This barely breaks the sub and can fail to
252+ Atk Bouffalant Return vs. 0 HP / 0 Def Serperior: 169-201 (58 - 69%) -- guaranteed 2HKO
and if you got the leaf storm boost
+1 252+ Atk Bouffalant Return vs. 0 HP / 0 Def Serperior: 255-301 (87.6 - 103.4%) -- 25% chance to OHKO
So again a solid counter to Serperior and a solid mon in RU(C rank viability despite its PU usage ranking) but what was once a rarity/niche mon(correct me if I am wrong) has risen up from 2 tiers below as a way to handle a common threat. Sounds like overcentralizing to me.
Bouff wasn't nominated to rise because it took on Serp. It rose because it's a devastating attacker that found more openings for a set up. Again, this "centralization" is more or less the metagame adapting.
So Fletchinder. Not sure why so many feel this is a counter as its one time at best if rocks are up due to its 4x weakness halving its HP but the move it quad resists does
252 SpA Life Orb Serperior Leaf Storm vs. 160 HP / 0 SpD Fletchinder: 81-95 (26.5 - 31.1%) -- guaranteed 4HKO
and if they predict Fletchinder as there is this counter mindset
252 SpA Life Orb Serperior Dragon Pulse vs. 160 HP / 0 SpD Fletchinder: 140-166 (45.9 - 54.4%) -- 57% chance to 2HKO
its practically popped if not straight up Ko-ed (with rocks up ofc)
But of course its a solid check as
252+ Atk Fletchinder Acrobatics (110 BP) vs. 0 HP / 0 Def Serperior: 282-332 (96.9 - 114%) -- 75% chance to OHKO
You Just oh-ko after serp attacked once(and had a 3/4 chance from full regardless) so it definitely checks but countering is a stretch imo. Conditional counter maybe? Is that a term now? Or should it be? hmmmm
The presence of SR when considering a Pokemon such as Fletchinder isn't reliable, as there will obviously be some means of removal. While I wouldn't say it (and perhaps Moltres) would be my first choice, defensive sets can definitely check Serperior with the right support.
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