This is the only part of your post I have anything to contribute on. I think Rotom-W is more troublesome than some give it credit for. Plenty of teams are willing to sacrifice something to stop volt-turn with rotom/landorus/scizor/talonflame/thundurus/zapdos/etc but for stall it isn't always so easy to just trash something to get your counter in for free. Rotom-W is by far the number one suspect, because it would be obvious to simply use a ground type to stop volt switch. However, his other STAB prevents most ground types from being viable switch ins. Pokemon with Volt Absorb or Motor Drive or Lightningrod generally don't appreciate the hydro pump either (or burns). You can't even trap the thing with Gothielle because it can Volt Switch out.
A simple Deoxys + Bisharp + Volturn + Offense can be a maddening battle for stall, with stacked hazards and possibly no chance to get the defog in (depending on your team's situation with Bisharp, oh my god it's hard to get Dark resist on the team). Nothing will be fast enough to catch the offensive pokemon in time and trying to switch your counters in wears everything down.
A full stop to Rotom-W is something I seriously want on all of my teams. I don't always fit it in when I'm throwing teams together, but it pays off enormously when I do because such pokemon can also generally take care of Thundurus, Zapdos, Raikou, and some water types on top of that.
Lanturn: Questionable, but I was able to put it to good use. Resists rotom's stabs, absorbs electric moves. Resists talonflames stabs as well. Also resists ice, and double resists steel. One big problem is if you run heal bell, you're running a cleric that is slow, no recovery, weak to EQ and solar beam, offensively weak, and only reasonably bulky. The HP stat can help make up for the defenses but wallbreakers can find their way through you. There's decent support options such as toxic, thunder wave, scald, discharge, rest, volt switch, rain dance, ice beam, icy wind, and soak. It's really only a pseudocleric though and its heal bell can't always be relied on.
Seismitoad: This thing is actually pretty cool. Rotom-W is stealth rock fodder. It doesn't like the burns of course, but you can legitimately go special with scald, grass knot, earth power, hidden power, or endeavor. None of these actually do anything to rotom, but you can use knock off, toxic, infestation, substitute, and/or rest to punish it. 105/75/75 defense is enough to be bulky on one side but even with full defensive investment Adamant CB talonflame still just slightly 2HKOs which is really goddamn stupid.
Gastrodon: We should all be familiar with this guy. Tons of support options, reliable recovery, and a great offensive movepool with nice base 92 stalling SpAtk. He and Seismitoad suffer a similar problem however, and that's lack of physical resistances. Things like close combat, secret sword, brave bird, earthquake, play rough, and return will do a number on you. Bulking up on the defensive side to protect against that will expose the low base SpDef. Gastrodon has another problem. He's not really doing anything to directly help his team. He's just sitting there in front walling what he's supposed to wall, but gastro won't be able to do anything against their switch in. He can't pass a wish, heal bell, control hazards, or anything like that. He just clings on with recover and tries to scald/toxic things.
These three pokemon crap on rotom-W and thundurus and plenty of other things, but lose to CB talonflame, lose to Charizard, and lose to specs keldeo. Something that water types just should not do unless there's a really good reason to have them on your team.
Any other ground types up to the task?
Zygarde: A truly interesting pokemon with better bulk, making him a better rotom switch in than garchomp or flygon.
108 HP / 121 Def / 95 SpDef
0 SpA Rotom-W Hydro Pump vs. 240 HP / 252+ SpD Zygarde: (22.3 - 26.13%) (you could set up a sub)
0 SpA Rotom-W Hidden Power Ice vs. 240 HP / 252+ SpD Zygarde: (32.61 - 39.32%) (holy crap lol)
252+ SpA Rotom-W Hydro Pump vs. 240 HP / 252+ SpD Zygarde: (30.45 - 35.97%) -- 2.93% chance to 3HKO after Stealth Rock
252+ SpA Rotom-W Hidden Power Ice vs. 240 HP / 252+ SpD Zygarde: (44.12 - 52.75%) -- 24.22% chance to 2HKO after Stealth Rock
252 SpA Life Orb Thundurus Hidden Power Ice vs. 240 HP / 252+ SpD Zygarde: (59.95 - 70.98%)
252 Atk Choice Band Mamoswine Ice Shard vs. 240 HP / 16 Def Zygarde: (80.57 - 94.96%) -- 6.25% chance to OHKO after Stealth Rock (wow!)
252+ Atk Choice Band Talonflame Brave Bird vs. 240 HP / 16 Def Zygarde: (47 - 55.63%) -- 80.08% chance to 2HKO after Stealth Rock (disappointing...)
252 Atk Choice Band Talonflame Brave Bird vs. 240 HP / 252 Def Zygarde: (35.49 - 41.96%) -- guaranteed 3HKO after Stealth Rock (thank thundurus for this)
252 SpA Choice Specs Keldeo Hydro Pump vs. 240 HP / 252+ SpD Zygarde: (47.72 - 56.35%) -- 89.06% chance to 2HKO after Stealth Rock (also disappointing...)
252 SpA Choice Specs Keldeo Hydro Pump vs. 240 HP / 252+ SpD Zygarde in rain: (71.46 - 84.41%) -- guaranteed 2HKO after Stealth Rock (this is nice at least)
252 SpA Choice Specs Keldeo Surf vs. 240 HP / 252+ SpD Zygarde: (39.08 - 46.28%) -- guaranteed 3HKO after Stealth Rock (the difference is astounding)
252 SpA Choice Specs Keldeo Secret Sword vs. 240 HP / 16 Def Zygarde: (41.72 - 49.16%) -- guaranteed 3HKO after Stealth Rock (this is huge)
252 SpA Choice Scarf Keldeo Icy Wind vs. 240 HP / 252+ SpD Zygarde: (43.16 - 51.79%) -- 10.94% chance to 2HKO after Stealth Rock (this is huge too)
252 SpA Choice Scarf Keldeo Hydro Pump vs. 240 HP / 252+ SpD Zygarde: (32.37 - 38.12%) -- 55.22% chance to 3HKO after Stealth Rock (fifty fifty)
252 Atk Choice Band Terrakion Close Combat vs. 240 HP / 16 Def Zygarde: (58.99 - 69.3%) -- guaranteed 2HKO after Stealth Rock (I fucking hate this pokemon)
252 Atk Terrakion Close Combat vs. 240 HP / 16 Def Zygarde: (39.08 - 46.28%) -- guaranteed 3HKO after Stealth Rock (ok)
252+ Atk Pure Power Medicham Hi Jump Kick vs. 240 HP / 16 Def Zygarde: (77.93 - 92.08%) -- guaranteed 2HKO after Stealth Rock (check!)
252 Atk Pure Power Medicham Drain Punch vs. 240 HP / 16 Def Zygarde: (41 - 48.44%) -- guaranteed 3HKO after Stealth Rock
252+ SpA Charizard-Y Fire Blast vs. 240 HP / 252+ SpD Zygarde in sun: (30.69 - 36.21%) -- 4.3% chance to 3HKO after Stealth Rock (that's more like it)
252 SpA Charizard Fire Blast vs. 240 HP / 252+ SpD Zygarde in sun: (28.05 - 33.09%) -- guaranteed 4HKO after Stealth Rock (even better!)
252 SpA Life Orb Sheer Force Landorus-I Earth Power vs. 240 HP / 252+ SpD Zygarde: (41.24 - 48.68%) (I like this)
0 SpA Life Orb Landorus-T Hidden Power Ice vs. 240 HP / 252+ SpD Zygarde: (42.44 - 51.07%) -- 5.08% chance to 2HKO after Stealth Rock (I like this too)
252+ Atk Life Orb Landorus-T Earthquake vs. 240 HP / 16 Def Zygarde: (50.83 - 60.19%) -- guaranteed 2HKO after Stealth Rock (Ouch...)
252 Atk Life Orb Landorus-T Earthquake vs. 240 HP / 16 Def Zygarde: (46.04 - 54.43%) -- 62.89% chance to 2HKO after Stealth Rock (Hmm...)
0 Atk Landorus-T Earthquake vs. 240 HP / 16 Def Zygarde: (30.21 - 35.49%) -- 0.63% chance to 3HKO after Stealth Rock (:D)
252+ Atk Bisharp Knock Off vs. 240 HP / 16 Def Zygarde: (34.53 - 40.52%)
252+ Atk Bisharp Sucker Punch vs. 240 HP / 16 Def Zygarde: (28.29 - 33.33%) -- guaranteed 4HKO after Stealth Rock
+2 252+ Atk Bisharp Iron Head vs. 240 HP / 16 Def Zygarde: (56.11 - 66.18%) -- guaranteed 2HKO after Stealth Rock
252 SpA Gengar Shadow Ball vs. 240 HP / 252+ SpD Zygarde: (23.74 - 28.05%) -- possible 4HKO after Stealth Rock (LOL)
252 SpA Life Orb Gengar Shadow Ball vs. 240 HP / 252+ SpD Zygarde: 129-152 (30.93 - 36.45%) -- 5.81% chance to 3HKO after Stealth Rock (meh)
252+ SpA Life Orb Aegislash Shadow Ball vs. 240 HP / 252+ SpD Zygarde: (36.69 - 43.88%) -- guaranteed 3HKO after Stealth Rock
252+ SpA Spooky Plate Aegislash Shadow Ball vs. 240 HP / 252+ SpD Zygarde: (34.53 - 40.52%) -- 99.51% chance to 3HKO after Stealth Rock
252 SpA Aegislash Shadow Ball vs. 240 HP / 252+ SpD Zygarde: (26.13 - 30.93%) -- possible 4HKO after Stealth Rock
0 SpA Aegislash Shadow Ball vs. 240 HP / 252+ SpD Zygarde: (22.3 - 26.13%) -- possible 5HKO after Stealth Rock
252 SpA Life Orb Deoxys-S Ice Beam vs. 240 HP / 252+ SpD Zygarde: (73.62 - 87.29%) -- guaranteed 2HKO after Stealth Rock (wtf)
252 SpA Life Orb Deoxys-S Psycho Boost vs. 240 HP / 252+ SpD Zygarde: (42.92 - 50.83%) -- 4.69% chance to 2HKO after Stealth Rock
4 Atk Life Orb Deoxys-S Superpower vs. 240 HP / 16 Def Zygarde: (21.82 - 25.89%) -- possible 5HKO after Stealth Rock (lol)
252+ SpA Choice Specs Politoed Ice Beam vs. 240 HP / 252+ SpD Zygarde: (90.16 - 106.47%) -- 75% chance to OHKO after Stealth Rock
0 SpA Politoed Ice Beam vs. 240 HP / 252+ SpD Zygarde: (45.08 - 53.71%) -- 40.63% chance to 2HKO after Stealth Rock (jesus christ it's not even likely to 2hko)
252 SpA Manaphy Ice Beam vs. 240 HP / 252+ SpD Zygarde: (59.47 - 70.02%)
252 SpA Manaphy Scald vs. 240 HP / 252+ SpD Zygarde in rain: (29.49 - 34.77%) -- guaranteed 4HKO after Stealth Rock
+3 252 SpA Manaphy Scald vs. 240 HP / 252+ SpD Zygarde: (48.92 - 57.55%) -- 96.09% chance to 2HKO after Stealth Rock (at least there's a chance!)
+6 252 SpA Manaphy Scald vs. 240 HP / 252+ SpD Zygarde: (77.69 - 91.6%) -- guaranteed 2HKO after Stealth Rock (I'm not sure how that works)
+3 252 SpA Manaphy Scald vs. 240 HP / 252+ SpD Zygarde in rain: (73.38 - 86.33%) -- guaranteed 2HKO after Stealth Rock
+3 252 SpA Manaphy Surf vs. 240 HP / 252+ SpD Zygarde: (54.91 - 64.74%) -- guaranteed 2HKO after Stealth Rock
+6 252 SpA Manaphy Surf vs. 240 HP / 252+ SpD Zygarde: (87.29 - 103.11%) -- 62.5% chance to OHKO after Stealth Rock (feeling lucky?)
+3 252 SpA Manaphy Surf vs. 240 HP / 252+ SpD Zygarde in rain: (82.25 - 97.12%) -- 25% chance to OHKO after Stealth Rock
252 Atk MegaPinsir Aerialate Return vs. 240 HP / 16 Def Zygarde: (49.16 - 58.27%) -- 98.83% chance to 2HKO after Stealth Rock (only Protect could save you)
+2 252 Atk MegaPinsir Aerialate vs. 240 HP / 252 Def Zygarde: (81.29 - 95.92%) -- 18.75% chance to OHKO after Stealth Rock (again I don't understand this)
+2 252 Atk MegaPinsir Earthquake vs. 240 HP / 252 Def Zygarde: (41.24 - 48.68%) -- guaranteed 3HKO after Stealth Rock
+2 252 MegaPinsir Aerialate Quick Attack vs. 240 HP / 252 Def Zygarde: (32.37 - 38.12%) -- 55.22% chance to 3HKO after Stealth Rock (Extremespeed bluffing)
252 SpA Life Orb Latios Draco Meteor vs. 240 HP / 252+ SpD Zygarde: (99.04 - 116.54%) -- guaranteed OHKO after Stealth Rock
252 SpA Latios Draco Meteor vs. 240 HP / 252+ SpD Zygarde: (76.25 - 89.68%) -- guaranteed 2HKO after Stealth Rock (punish the leftovers!)
252 SpA Life Orb Latias Draco Meteor vs. 240 HP / 252+ SpD Zygarde: (88 - 103.59%) -- 62.5% chance to OHKO after Stealth Rock
252 SpA Life Orb Latias Dragon Pulse vs. 240 HP / 252+ SpD Zygarde: (58.03 - 67.86%) -- guaranteed 2HKO after Stealth Rock
4 SpA Latias Dragon Pulse vs. 240 HP / 252+ SpD Zygarde: (35.97 - 42.2%) -- guaranteed 3HKO after Stealth Rock (finally a nice calc again)
4 SpA Latias Draco Meteor vs. 240 HP / 252+ SpD Zygarde: (54.67 - 64.74%) --> (27.81 - 33.09%) -- possible 2hko after Stealth Rock (not sure how to calc this)
0 SpA Clefable Moonblast vs. 240 HP / 252+ SpD Zygarde: (23.5 - 27.81%) -- possible 4HKO after Stealth Rock (coil up or toxic!)
252+ SpA Life Orb Clefable Moonblast vs. 240 HP / 252+ SpD Zygarde: (42.44 - 50.59%) -- 2.34% chance to 2HKO after Stealth Rock (:D)
252+ Atk Choice Band Tyranitar Crunch vs. 240 HP / 252 Def Zygarde: (36.69 - 43.4%) -- guaranteed 3HKO after Stealth Rock
252 Atk Tyranitar Ice Punch vs. 240 HP / 252 Def Zygarde: (65.22 - 76.73%) -- guaranteed 2HKO after Stealth Rock (mother of god)
252 Atk Garchomp Outrage vs. 240 HP / 16 Def Zygarde: (79.13 - 93.52%) -- guaranteed 2HKO after Stealth Rock
252 Atk Dragonite Outrage vs. 240 HP / 16 Def Zygarde: (81.05 - 95.44%) -- 12.5% chance to OHKO after Stealth Rock
0 SpA Goodra Ice Beam vs. 240 HP / 252+ SpD Zygarde: (50.83 - 60.43%) -- guaranteed 2HKO after Stealth Rock
252+ SpA Goodra Ice Beam vs. 240 HP / 252+ SpD Zygarde: (69.06 - 81.53%) -- guaranteed 2HKO after Stealth Rock
0 SpA Goodra Dragon Pulse vs. 240 HP / 252+ SpD Zygarde: (35.97 - 42.2%) -- guaranteed 3HKO after Stealth Rock
252+ SpA Goodra Dragon Pulse vs. 240 HP / 252+ SpD Zygarde: (48.92 - 57.55%) -- 94.92% chance to 2HKO after Stealth Rock
252+ SpA Goodra Draco Meteor vs. 240 HP / 252+ SpD Zygarde: (79.61 - 94%) -- 6.25% chance to OHKO after Stealth Rock
252 SpA Greninja Protean Ice Beam vs. 240 HP / 252+ SpD Zygarde: (89.2 - 106.47%) -- 68.75% chance to OHKO after Stealth Rock (eh, some sets don't use LO)
0 SpA Zapdos Hidden Power Ice vs. 240 HP / 252+ SpD Zygarde: (37.41 - 45.08%) -- guaranteed 3HKO after Stealth Rock (I don't know what SpA EVs are used)
252 Atk Life Orb Excadrill Earthquake vs. 240 HP / 16 Def Zygarde: (43.88 - 51.79%) -- 14.84% chance to 2HKO after Stealth Rock
252 Atk Excadrill Earthquake vs. 240 HP / 16 Def Zygarde: (33.81 - 39.8%) -- 95.85% chance to 3HKO after Stealth Rock
176 Atk Excadrill Earthquake vs. 240 HP / 16 Def Zygarde: (32.37 - 38.12%) -- 55.22% chance to 3HKO after Stealth Rock (an assault vest set?)
252 Atk Infernape Close Combat vs. 240 HP / 16 Def Zygarde: (33.81 - 39.8%) -- 95.85% chance to 3HKO after Stealth Rock
STAB Ice Beams, LO Ice Beams, Specs Ice Beams, and Certain Band/Specs/LO Outrages/Meteors are just about the only thing that can OHKO him
252+ SpA Togekiss Air Slash vs. 240 HP / 252+ SpD Garchomp: (22.54 - 26.85%) -- possible 5HKO after Stealth Rock -- You can sub up on the hax togekiss!
Alright, clearly this guy will have his opportunities. These are the top threats in the meta and it's not like he has to try to tank all of this stuff on his own, I just listed these out to show the serious things he can survive, and the things that are slightly too much for him. I mean half the time he can switch into a harmless rotom or charizard or something. All of the above calcs included Leftovers, so I'm not sure what'll happen if you want to give him Sitrus berry so he can handle Pinsir and Specs Pump Keldeo. In a way, Sitrus is equal with 4 turns of leftovers so it might not be all bad. Anyway, how about we get some steels and water types to help him out, and let Zygarde do his own stally thing?
Zygarde @ Leftovers
Ability: Aura Break
EVs: 240 HP / 252 SDef / 16 Def
Careful Nature
- Rest
- Coil/Sleep Talk/Earthquake
- Coil/Sleep Talk/Earthquake
- Dragon Tail
Zygarde @ Leftovers
Ability: Aura Break
EVs: 240 HP / 252 SDef / 16 Def
Careful Nature
- Rest/Protect
- Toxic/Glare/Haze
- Toxic/Glare/Haze
- Dragon Tail
Zygarde @ Leftovers
Ability: Aura Break
EVs: 240 HP / 252 SDef / 16 Def
Careful Nature
- Toxic/Glare
- Dragon Tail
- Rock Slide
- Earthquake
The first set is incredibly tempting to run something like the crocune, but beware of fairies if you choose not to run earthquake. Zygarde is one of those pokemon that can actually get away with resting without necessarily using sleep talk, remember that. Also remember that sleep talk fails 33% of the time anyway and is only useful during the two turns of rest.
The second set tries to be a good teammate by increasing it's own longevity while also messing with opponent pokemon. Rest is mentioned again because it's the only chance at recovery. Remember, there are plenty of pokemon you can freely rest against, like bulky Rotom-W, Skarmory, Charizard-Y, Chansey, bulky Aegislash, Gengar, and Gliscor.
The third set aims to retain the bulky dragon tail while also having a direct answer for fire/flying and bug/flying types and bug/fire. A status move of your choice.
To be honest basically any logical combination of Rest, Protect, Sleep Talk, Toxic, Coil, Glare, Haze, Dragon Tail, Earthquake, Substitute, Bind, and Rock Slide would be a good stall mon.