Stuff that needs to go:
Terrakion. What counters does this thing actually have? Not checks, Counters. Rockium Z Stone Edge shreds the tier at +2, Close Combat nails the few things that can take it, it's fast and has OK bulk. Get it out.
Serperior. Why is this thing legal? Thanks to Contrary you get a 130 BP STAB Nasty Plot, it's very fast and it's no one trick pony. It's got Glare to cripple fast frail mons, Taunt to shut down walls, it can run an AoA set and a defensive one equally well. It's very fast as well.
Weavile. It's safe switch ins are pretty much Keldeo and Primarina, both of whom hate losing their items due to Knock Off. It has STAB priority as well, so you can't revenge it with faster stuff.
Defensive counters to Terrakion not named Palossand are a bit lacking, certainly...But Hippowdon exists, ya know:
+2 252 Atk Life Orb Terrakion Close Combat vs. 252 HP / 252+ Def Hippowdon: 325-383 (77.3 - 91.1%) -- guaranteed 2HKO after Stealth Rock and Leftovers recovery
And Mega Slowbro still takes +2 Rockium Z Stone Edge if you run a fair amount of Defense:
+2 252 Atk Terrakion Stone Edge vs. 252 HP / 140+ Def Slowbro-Mega: 291-343 (73.8 - 87%) -- guaranteed 2HKO after Stealth Rock
By that extent, it goes without saying that you handle LO Terrakion too, even without 140 Defense:
+2 252 Atk Life Orb Terrakion Stone Edge vs. 252 HP / 80+ Def Slowbro-Mega: 220-259 (55.8 - 65.7%) -- guaranteed 2HKO after Stealth Rock
That's much more Defense than Mega Bro typically runs (far as I know), but if you're that worried about Terrakion, it gets the job done. That aside, in the current metagame as offensive as it is, it's pretty easy to keep Terrakion in check offensively...You have threats that carry priority and can easily revenge it in Azumarill, Scizor, Breloom, Infernape, and Conkeldurr, threats that naturally outspeed or at the very worst, tie with Terrakion and force it to gamble in the likes of Keldeo, Serperior, Cobalion, Weavile, Starmie, Latias, Thundurus, Mega Aerodactyl, and Gengar, to name a few. Lastly, Choice Scarf users such as Keldeo, Thundurus-Therian, Staraptor, and Salamence can also revenge kill it with ease. All of these Pokemon I listed are relevant threats in the meta, and you're likely to see some of them on every team you face. This is how Terrakion is kept in check...don't get me wrong it shreds defensive teams like nobody's business, but with all the threats we have to contend with, the defensive playstyles that Terrakion excels against are generally tougher to pull off anyway.
This same exact argument exists for Serperior, though it's a bit harder to revenge kill becauase of that Base 113 Speed stat. Still, sets lacking HP Fire 100% lose to Scizor, base 115s and above like Starmie, Tornadus-T, Raikou, etc. can still take on Serperior. Defensively
AV Dhelmise, SpD Decidueye, Mandibuzz, Klefki, and Blissey all work against Serperior.
Also Azumarill is a pretty solid switch-in to Weavile, especially if you run Z-Belly Drum, because that means Azumarill is immune to Knock Off and can set up on Weavile so long as it lacks Poison Jab.
