Alright, Serperior is ridiculous and it has a maximum of 2
good answers at most, both of which can't really be called
good answers because they simply would not be considered for a team if serperior was not in the tier, those two being SpDef Roserade and Amoonguss. If you want something to beat fighting types, there are much better options than Amoonguss in the tier (Florges, Crobat, Granbull and Nidoqueen are included in this argument). I have not seen a single amoonguss in months, despite the fact that I play the tier almost daily. This is exactly what happens when broken mons are introduced into the tier: people use pokemon that otherwise would not be considered to check that specific pokemon (see Slowking / Doublade in Mega Medicham and Staraptor meta, huge rise in Whimsicott during the Zygarde meta). I don't really like using roserade in general, but SpDef roserade, at least in my opinion, is generally an inferior set of a decent-at-best pokemon, mostly due to the fact that it isn't even able to check Suicune of any kind, which is the only reason I would use Roserade in the first place. The other options that have been mentioned are:
Forretress: Yeah no. Forretress is a terrible option which actually
lets serperior set up. Serperior has a ridiculous amount of bulk for such a potent setup sweeper, allowing it to take 76 BP Gyro Balls easily. In addition, standard Physically Defensive Forretress takes ~20-24% from Life Orb boosted leaf storm
initially, and what is obviously the most broken part of Serperior is how much the damage increases the longer it stays in: that damage doubles up to 40-48% after +2, which means your Forretress will be at around 40-50% after 2 leaf storms and leftovers recovery assuming you're switching into serperior with your forretress, and it only deals 41-48% with Gyro Ball. Therefore, Serperior is going to be sitting at around 35% after 2 rounds of LO recoil. If it runs Synthesis, Forretress is in an even worse condition, since it gives Serperior a free +6 and leaves it at about 50% when forretress dies instead of 25%. Also to
reachzero's post: Gyro Ball 2HKO's? This is the most damage you're going to get out of Gyro Ball onto a Timid Max Speed Serperior:
252 Atk Forretress Gyro Ball (117 BP) vs. 0 HP / 0 Def Serperior: 156-184 (53.6 - 63.2%) -- guaranteed 2HKO
So Forretress is a very, very shaky check at best, and I would hesitate to even call it a check. And, if you run 252 attack, you're either giving up your HP investment which means you take even more from Leaf Storm, or you're giving up your defense investment, which makes forry less effective outside of checking Serperior.
Moving onto Goodra: Goodra is the only viable Sap Sipper user in this tier, and even it is worn down by Dragon Pulses trying to switch in. Also, CB Goodra is the only Goodra set that should ever be run, as AV goodra is really, really weak and shut down by pretty much any defensive pokemon in the tier. Again, between Dragon Pulse, entry hazard (if you have them up) and residual damage goodra is a
very shaky check but it can serve as one.
On the other hand, we have multiple offensive checks that can't switch in, including Crobat (fear of Glare), Mega Aerodactyl (Leaf Storm does 64-76% to 40 HP Mega Aero), Mega Beedrill (Fear of Glare, gets hit hard by Dragon Pulse and HP Fire, and loses 1/4 of its health to a quad resisted Leaf Storm), Choice Scarf Darmanitan and Infernape (again, Glare and Dragon Pulse / Leaf Storm; Leaf Storm does 44-51% to Darmanitan unboosted with a LO and a little more to Infernape, 2hkoing both), and Fletchinder, which doesn't appreciate a LO Dragon Pulse, taking almost half.
Far and away the biggest issue with Serperior to me is that it causes issues for
every playstyle considering all of the possible movesets it can run. Glare screws with offense and offense lacks a true switchin, as everything that doesn't care about being paralyzed (not much) is destroyed by LO or Meadow Plate boosted Leaf Storms, and Serperior outspeeds most of the stuff on standard offense as well with its ridiculous speed tier. Balance dislikes Taunt Serperior, as it's capable of shutting down all of the defensive answers to Serperior easily. The offensive and support pokemon on Balance which can deal with Taunt Serperior dislike Glare, as stuff like Crobat, Entei, and Mega Aerodactyl can all be crippled on switch-in. Balance has it the easiest though, and stall has it the hardest; stall is basically forced to run SpDef Roserade, Amoonguss or AV Goodra (none of which stall wants to run) or it's an automatic loss vs Taunt + Synthesis Serperior which can blow it's way through stuff like Blissey with ease. We have never seen a pokemon so adept at handling every playstyle as easily as Serperior does and so capable of ripping its way through entire teams with just one move. The downside of setup sweepers is that they have to try to find a turn to setup, which means taking chip damage, getting statused, or possibly getting phazed out if they want to try and set up; Serperior is able to work as a revenge-killer, a wallbreaker, and a setup sweeper all with one move. The ridiculousness of the moveset of Serperior is that it has 2 free slots, in my opinion; all it has to run is Leaf Storm + Dragon Pulse, and in the last 2 slots it can run any combination of Taunt, Synthesis, Leech Seed, Substitute, Glare, HP Fire, and HP Ice, all of which give it ways to get around its few checks.
so tl;dr for those of who you didn't bother to read my endless droning: serperior has few answers in the tier at all, and those answers it does have are shaky at best, whether it be checking serperior or doing well in the tier at all; this list is restricted to spdef roserade and defensive amoonguss, with the possible inclusion of av goodra, while all other answers cannot switchin due to fear of leaf storm or a coverage move. serperior is easily able to handle any playstyle currently effective in the UU metagame and thus it should be
banned.
good luck to those of you shooting for reqs!