Bringing a pretty nice defensive backbone to the party, featuring two rather uncommon mons and one pretty unconventional set. Threats that are accounted for and not accounted for at the bottom.
Garchomp-Mega (M) @ Garchompite
Ability: Rough Skin
EVs: 252 HP / 176 Def / 76 SpD / 4 Spe
Impish Nature
- Earthquake
- Dragon Tail
- Rest
- Sleep Talk/Toxic/Fire Blast/Stone Edge
Now, Mega Garchomp is normally seen as a super-powerful wallbreaker (4 attacks set, can be mixed) or a sweeper (SD). But what's often overlooked is that very nice bulk (108/115/95) with a defensive typing that lets it beat a lot of common Volt-Turn members, and properly EV'd, lets this check/counter many of the top threats in the meta. Put this thing in balanced/Defensive teams in need of something that walls Charizard Y, Bisharp, and Heatran all in one slot, while checking/countering a bunch of miscellanous top-tier OU mons. In addition, having a sleep absorber is nice. While similar to TankChomp in many ways, this set benefits from the greatly increased bulk and the massive increase in damage (you still hit harder than ScarfChomp with 0 attack investment), so you still maintain quite a great offensive presence (mentioned later) while being fat as hell.
EQ for powerful STAB, Dragon Tail for STAB phazing and racking up hazard damage, Rest because it's your only form of recovery, and Sleep Talk because you now function as a great status/sleep absorber. Toxic is okay, as are Fire Blast (for Ferro and Mega Scizor who wall or can set up on you depending on the spread), Stone Edge is pretty much for SpDef/Stallbreaker Talonflame. 4 EVs to outspeed defensive Rotom-W and Lando-T, rest in bulk. Calcs can be found in the "Creative and underrated sets" thread, showing how bulky this thing is.
Bronzong @ Leftovers
Ability: Levitate
EVs: 252 HP / 4 Def / 252 SpD
Sassy Nature
IVs: 0 Spe
- Gyro Ball
- Earthquake
- Toxic
- Protect/Stealth Rock/HP Ice
Bronzong handles most of the fairies that plague MegaChomp, while acting as a fairly solid special wall. EVs are standard, as are Gyro Ball (dumpsters fairies and fast frail mons) and to a lesser degree, EQ (for Metagross and to surprise Heatran and non-Air Balloon Excadrills). Toxic is good for hitting stuff like Hippo which you can't damage, and putting a timer on Lando-I/Nasty Plot Thundurus, etc. Protect is useful in conjunction with Toxic and Leftovers, but it also functions to scout out coverage moves that might threaten Bronzong or the core. Stealth Rock is also a great option since Bronzong is pretty good at setting them up and beats some common SR setters like Mamoswine. HP Ice lets you beat Garchomp/Lando-I/Gliscor but isn't necessarily good simply because you'll not OHKO those pokemon with no investment. If you run Ice Beam on Clefable as shown below, no need to run HP Ice (which in all honesty is probably suboptimal anyways).
Clefable @ Leftovers
Ability: Unaware
EVs: 252 HP / 172 Def / 84 SpD
Calm Nature
- Moonblast
- Moonlight
- Heal Bell
- Ice Beam
Clefable's ability to handle most Kyurem-B sets makes it a pretty nice partner to the core, which struggles with Kyurem (both forms) pretty hard due to Teravolt negating Bronzong's Levitate and Ice STAB wrecking Mega Garchomp. It also handles the two common variants of MSableye, which doesn't really care about Bronzong and this Garchomp set. Another threat handled pretty nicely by this and not as much by the rest of the core is the relatively uncommon SD Mega Heracross. Standard-ish EVs, if you want to handle LO Tornadus T/Kyurem-B/TG Manaphy better, give more SpDef investment. Heal Bell is great because Toxic ruins Unaware Clefable, and being able to wake up MegaChomp in a pinch is really useful (and Bronzong really doesn't like burns since they neuter Gyro Ball and EQ, but more because it's chip damage). Ice Beam seems weird, but it lets Clefable beat SD Gliscor (and other variants too), who otherwise walls this core hard and can set up on it pretty easily.
Threats Covered: Life Orb Gengar (hard countered by MegaChomp), Charizard Y (hard countered by MegaChomp), Mega Metagross that don't carry Ice Punch (countered/checked by MegaChomp and Bronzong, depending on moveset), Life Orb Mamoswine (soft countered by Bronzong, hard assuming no Knock Off), Life Orb Excadrill (hard countered by MegaChomp), Offensive Heatran (hard countered by MegaChomp), Mega Gardevoir (soft countered by Bronzong), Choice Band Victini (hard countered by MegaChomp), Mega Lopunny (checked/countered by MegaChomp if no Ice Punch, checked/countered by Clefable otherwise), (SD) Mega Gallade (checked/countered by MegaChomp if no Ice Punch, checked countered by Clefable otherwise), (SD) Mega Heracross (countered by Clefable), Life Orb Bisharp (hard countered by MegaChomp), Choice Band Scizor (hard countered by MegaChomp), Scarf Keldeo (soft countered by MegaChomp and Clefable), Scarf/LO Kyurem-B (Checked/Countered by Clefable depending on the moveset and spread of Kyurem-B), Life Orb Breloom (checked by MegaChomp/Bronzong/Clefable), Calm Mind Mega Sableye (countered by Clefable), SD Gliscor (countered by Clefable), Mega Gyarados (hard countered by MegaChomp if no Ice Fang, hard countered by Clefable assuming no Iron Head), Choice Specs Dragalge (checked by MegaChomp, countered by Bronzong), Mega Manectric (checked by MegaChomp and Bronzong), (Mega) Latios and Latias (countered by Bronzong and checked/countered by Clefable), Choice Band/DD/Scarf (Mega) Tyranitar (countered by Mega Chomp if no Ice Beam, checked/countered by Clefable otherwise), Mega Pinsir (checked by Bronzong and by Clefable/MegaChomp), Specs Raikou (checked by Bronzong and MegaChomp), Mega Diance (countered by Bronzong, checked by MegaChomp), Physical/Cotton Guard Mega Altaria (checked by Bronzong, countered by Clefable), Banded Talonflame (countered by MegaChomp)
Threats not covered: (TG) Manaphy, NP Thundurus, CM Landorus-I, AoA Landorus-I (checked by Bronzong and MegaChomp, but not as well as I'd like), Mega Metagross with Ice Punch, LO Tornadus-T, BD Azumarill, CroBro/Cune, Offensive Mega Venusaur (MegaChomp checks it and Bronzong kinda walls it if no HP Fire but that's it), Mega Medicham, Volcarona, Special Mega Altaria, Specs/CM Keldeo, Stallbreaker/SpDef Talonflame
Obviously, haven't listed everything, but this shows how darn versatile this defensive core is. Good defensive partners will address those threats, and good offensive partners include strong sweepers that can muscle past Ferrothorn especially (who just walls these 3 fairly casually).