Hidden Power Fire is great because Amoonguss is a good check to Mega Mawile as long as Sleep Clause hasn't been activated, and it would be a shame to not be able to do a thing to SubPunch Mega Mawile. It is also great so that Ferro can't come in for free to absorb Spore and set up hazards, and in general gives valuable coverage against many threatening Steel-types, such as Mega Scizor, Bisharp, and the aforementioned Mega Mawile. Not to mention that Aegislash 2HKOes with SR up anyway with Shadow Ball, so i don't see the appeal in Foul Play. Spore + Giga Drain + Clear Smog are musts, the first to be able to actually threaten things, the second to deal with a lot of the stuff that Amoonguss checks (Mega Gyarados, Keldeo, Azumarill, Terrakion) and have a reliable STAB, and Clear Smog to counter Belly Drum Azumarill if Sleep Clause has been activated, which could otherwise tank even one Sludge Bomb (after Sitrus and no SR) and 2HKO, and to check CM Clefable. So Amoonguss has only one slot for customization, where Hidden Power Fire and Hidden Power Ice are the best options imo, and Sludge Bomb is decent too.
If Sleep Clause hasn't been activated, then Mega Mawile can just switch out and sleep fodder something else, and the next time you come in it can predict your Hidden Power Fire and eventually wear you down if you stay in for longer than two turns. It also isn't a good (or good enough) check to SD Mawile since most of them will have at least one of Iron Head, Fire Fang, or Knock Off, the first two OHKOing you after a Swords Dance and the last 2HKOing you after a Swords Dance, and again since it only takes one turn for Mawile to set up sleep foddering something else isn't that big of a deal.
Mega Scizor you can't really beat even with Hidden Power since you could easily be set up fodder for it, giving it chances to Swords Dance or recover, or if it Swords Dances while you switch you are still 2HKOed by it while Hidden Power Fire only 2HKOs 34% of the time, and after a Swords Dance Foul Play is a guaranteed 2HKO and if it's a variant with U-turn it can U-turn out and let something else take the Hidden Power or Spore, while most switchins to Amoonguss will take at least more damage from Foul Play than from HP Fire because most of them are either Psychic types or physical Flying types. Ferrothorn is the best reason for HP Fire, so I'll give you that, but otherwise it's mostly worse than Foul Play.
Aegislash 2HKOs physically defensive Amoonguss after Stealth Rock with Shadow Ball/Flash Cannon most of the time, yes. But if the Aegislash user uses King's Shield, Swords Dance, Shadow Sneak, or Sacred Sword on the switch, then it's a dead or sleeping Aegislash, unless it switches out, which in that case Amoonguss will still have Foul Play to hit it later, unlike Hidden Power Fire Amoonguss who only 3HKOs Aegislash-Blade with HP. Foul Play is also another one of Amoonguss's answers to Swords Dance/Dragon Dance/Bulk Up users, hitting Psychic-types for fair damage on the switch-in, and hitting Pokemon that just have high physical attack such as Kyurem or something on the switch-in. Clear Smog's only real purpose is for BD Azumarill, but if Sleep Clause is assumed to not be activated for something like Mega Mawile, who can usually switch out because it only takes one turn to set up with no drawbacks, then I don't get why it's going to be activated for something like BD Azumarill, who only really gets one chance to set up the whole game or is able to be killed be any attack if it sets up twice, so it won't want to switch out as much. However, Clear Smog is usually better, but Stun Spore and Hidden Power Fire could probably both be used if needed. CM Clefable probably shouldn't be mono-attacking, and Spore gets it anyways if it's a real problem to your team. Anyways, Foul Play has been and IMO still is the best option for that slot, as if you're using Amoonguss over Mega Venusaur you should probably be using a Fire-type alongside it.
Every single Slowbro should be carrying Thunder Wave and a paralyzed Mega Charizard X is a useless one, at least offensively. Also, Thunderpunch is not a thing and Slowbro can tank one +1 Adamant Outrage and use Thunder Wave, so no matter what Slowbro hard counters it.
A paralyzed Mega Charizard is just as useless as one confused or something. Counters shouldn't be based off of luck, so I don't get how Thunder Wave makes Slowbro a hard Charizard counter. A paralyzed Mega Charizard X still outspeeds a Slowbro, and assuming you don't get fully paralyzed it kills Slowbro. I was under the impression that Thunder Punch was kind of a thing for getting Azumarill, but okay.
All out attacking sets don't usually run Taunt but SR, so Slowbro hard counters them. Taunt is found on leads that are either Rain Dance or Dual Screens sets. Slowbro is a great check to CB Azumarill and that's it, i never said it does anything against BD sets. As long as you have a bulky Fairy-type resist alongside Slowbro, CB Azumarill should never be a problem.
Maybe not usually, but they could if they wanted to or if Slowbro became a serious problem for them. Like I said, they could go mixed if they wanted to as well.
You don't use Thunder Wave against Dragonite but Ice Beam, which 2HKOes even through Multiscale, and if Dragonite uses Outrage Slowbro can just switch into one of its Dragon-resisting teammates to finish off Dragonite. Slowbro doesn't counter Dragonite without SR up, but it's a pretty solid check.
4 SpA Slowbro Ice Beam vs. 4 HP / 0 SpD Multiscale Dragonite: 130-154 (40.1 - 47.5%) -- guaranteed 3HKO
Does Slowbro usually run more Special Attack investment than that or something?
Slowbro walls Hippo and can burn it back, and then spam Scald until Slack Off is over of PPs if Hippo is specially defensive. This is a counter. Slowbro counters Terrakion it doesn't check it, you must have the terms mixed up:
- +2 252 Atk Life Orb Terrakion Stone Edge vs. 252 HP / 252+ Def Slowbro: 286-337 (72.5 - 85.5%) -- guaranteed 2HKO after Stealth Rock and Leftovers recovery
While Slowbro paralyzes back and then can finish off Terrakion with Scald. Even if Slowbro dies in the process because Scald doesn't OHKO, the opponent will be left with a ~10 health left paralyzed Terrakion, which means that Slowbro has countered Terrakion. Mandibuzz can't touch you but you can burn it, with in combination with SR, makes her incredibly easy to wear down. I was talking about SR Skarmory, which is by far the most common hazard used on Skarmory, and one a 1 v 1 scenarion of SR Skarmory and Slowbro, SLowbro always comes on top, i hope i don't have to explain how.
Hippo can Toxic Slowbro, set up Stealth Rock, Slack Off, and spam Earthquake until it dies. On a strict 1 v 1 Hippowdon wins, but Slowbro will win if it switches out and has cleric support, although Hippowdon will probably have a cleric as well. Any other Hippowdon set Slowbro beats I guess, but most Hippowdon aren't offensive. Again you're still relying on luck to beat Terrakion, it definitely is a check, but the only other non-Scarf set isn't Swords Dance. There's also leads with Taunt and Choice Band, which Slowbro checks but does not counter. You didn't specify SR Skarmory, and Spikes is common as well and better IMO since there are about 1000000 other defensive Pokemon with Stealth Rock but Skarmory, Chesnaught, and Ferrothorn are some of the few with Spikes. While Skarmory can't deal with Slowbro, it can force it out, and while that means Slowbro won't be taking any damage from the hazard, the rest of your team still will be. It's not like dealing with Skarmory is one of the grand achievements that every wall wishes to have either.
No, the lists are the ones i initially mentioned, just without Aegislash on the list of Amoonguss, as i already showed you multiple times that Amoonguss gets 2HKOed by Shadow Ball with SR up. But really, if by your definitions Slowbro is a soft check to Mega Char X, you either need to change them to match those that the rest of us are using or lurk more.
While Slowbro can check Charizard X, it isn't a counter like you're saying. If you really need a counter to Charizard then you're better off using Hippowdon or Quagsire or something, not Slowbro. Slowbro checks non-Outrage Mega Charizard X provided that Charizard gets at least one turn of full paralyzation and the Slowbro user outpredicts the Charizard user by using Scald, Slack Off, and Thunder Wave at the right moments. Since most Charizards aren't Outrage, and since the best play is assumed, Slowbro does check Mega Charizard X. Outrage 2HKOing it and its reliance on Thunder Wave if Charizard has Outrage is what makes it a check and not a counter. Slowbro is also easier to switch into compared to Amoonguss because Amoonguss has the threat of Foul Play and Spore, which is why I think Amoonguss is worth being in the same rank as Slowbro. Amoonguss has many utilities that make it equal to Slowbro, such as never being useless in any game and more customization based on what threatens your team, which Slowbro does not have.