I'm posting 2 answers because I have too many characters for 1 answer. This is for Pure Hackmons.
If you have no clue what to put on your Pure Hackmons Pokemon set, Neutralizing Gas and Safety Goggles are pretty good defaults. As mbouchon says, Safety Goggles gives you immunity to Spore (Spore is the only viable move Safety Goggles blocks, though), possibly buying you crucial turns against Shell Smash sweepers and luring opposing Knock Offs. Due to turning off nearly all opposing abilities, Neutralizing Gas lets you switch into opposing No Guard mons (not without risk of being OHKOd the old-fashioned way), forces both them and Wonder Guard mons to suppress your ability, and lets you ignore opposing Magic Bounce (useful on hazard setters, sleep spammers, Taunters, and Encorers) and Sturdy (useful on mons with OHKO moves). As an additional gameplay tip, if you predict the Gastro Acid from a suspected faster No Guarder, firing off an OHKO move is a great way to juke them.
This leads into this gameplay tip: always assume that any Pokemon with no ability message on switch-ins and no Air Balloon has No Guard as its ability. This means that switching in any Pokemon without Neutralizing Gas, Sturdy, or Wonder Guard with no current weaknesses to Normal, Ground, and Ice will probably lead to your mon being OHKOd. This also means that, if the opposing suspected No Guarder suppresses your ability, you should switch out or Dynamax the next turn. As a corollary, switching your No Guarder into any mon without Neutralizing Gas that hasn't been completely scouted for OHKO moves is rather risky, especially if you're switching out OHKO bait such as Eternamax.
Similarly, any mon that successfully uses Lock-On or Mind Reader will probably use a OHKO move the next turn, so make sure to switch out or Dynamax.
Leads often stray from using Safety Goggles because Lovely Kiss, Taunt, and even Trick/Switcheroo are more common on leads, due to dodging Safety Goggles and thus being usable on Turn 1. Lum Berry lets you dodge Lovely Kiss (and Nuzzle/Glare) once, while Mental Herb lets you dodge Taunt/Encore once.
If your team includes a Shell Smasher, scouting the opposing Eternamaxes' overflow-jacked defences is pretty good and will often let you not waste turns hitting Eternamaxes for almost-zero damage late-game. Make sure to keep track of which moves an Eternamax hit by a move for over 1% damage used. Mousing/hovering over that Eternamax should uncover what moves that Eternamax used (even if the game is uncertain which Eternamax it is and therefore assigns more than 4 moves to it), thus helping you determine which defence(s) is not the overflowed defence on that Eternamax. Disruptive moves like Knock Off, Rapid Spin, Lava Plume, Anchor Shot, Thunder Cage, Dragon Tail, Circle Throw, and Clear Smog are often sufficient for determining jacked Eternamax defences, but Spectral Thief is especially valuable for bypassing Substitute as well as stealing opposing stat boosts, thus letting you scout opposing Eternamax jacked defences through Substitute.
Shell Smashers that use Spore need Knock Off support, and without ensuring that all remaining opposing mons have no Safety Goggles and are not Comatose, Spore users risk either losing turns hitting Spore-immunes or not using Spore and leaving themselves open.
Speed tiers in Pure Hackmons can get a little weird. Notably, Eternamax needs a speed-boosting nature and +1 in Speed or a Choice Scarf to outspeed Regieleki (or +2 in Speed). This means that Scarf Eternamax either sacrifices its ability to deal with Regieleki or its ability to take hits. Additionally, any mon that underspeeds min Speed, Speed-lowering nature, 0 Speed IVs Eternamax and therefore Shell Smashes after its Spectral Thief fails to outspeed Regieleki after 1 Shell Smash. (I personally recommend outspeeding Regieleki after 1 Shell Smash. Regieleki often packs really annoying disruptive moves such as Lovely Kiss or Trick.) Shell Smashers that speed tie with or are faster than Eternamax (e.g. Calyrex-Shadow) should use a disruptive move that prevents Eternamax from connecting with Spectral Thief (e.g. Spore, Entrainment Normalize, Imprison Spectral Thief). Shell Smashers slower than Eternamax (e.g. Rayquaza, underspeed Mewtwo) can afford to use coverage. Due to how squishy Indeedee and Obstagoon can be despite their immunity to Spectral Thief and their need to boost multiple times to sweep, they often need to use disruption (e.g. Spore, Substitute, Taunt) instead of coverage.
Precisely because Shell Smashers often forgo coverage for disruption, Dark-types are viable for completely blocking some Psychic-type Shell Smasher movesets and often resisting Power Trip. I recommend Magic Bounce or Comatose for also hosing Entrainment Normalize Calyrex-Shadow, although Techno Blast can sting. A Dark-type with an immunity to Ground/Normal such as Yveltal or Spiritomb has some utility against teams with no Shell Smashers because it can switch into OHKO moves, although Spiritomb loses to Obstagoon, especially when switched into Obstagoon.
Because opponents will try hazard stacking (especially Comaphazer-using opponents), I personally believe that packing a hazard-controlling move is mandatory. Rapid Spin is probably the most common one because you can use it while Taunted, it removes only hazards on your side, it speed boosts, and it has an insanely high 64 PP (thus letting you PP stall in emergencies), although I'm sure Court Change is viable.
Be aware that any plan that takes too long to KO an opponent leaves you open to OHKO moves. This is why slower sets like the Perish Song Trapper and the Leech Seed Taunt Trapper often use Substitute.
Your team should hopefully have multiple plans for dealing with opposing Substitute. Substitute punishes OHKO moves quite hard, although you can still use OHKO moves in desperation. Phazing moves remove Subs, while strong enough regular attacks (e.g. Bolt Beak, Anchor Shot, Knock Off, Lava Plume, Double Iron Bash, Precipice Blades, Thousand Arrows) can wipe out opposing non-Eternamax Subs, Night Shade/Seismic Toss/Nature's Madness wipes out any Sub in 1-2 shots (Nature's Madness is unreliable when your opponent has little HP), Spectral Thief, Ghost Curse, and Perish Song bypass Sub (and so does Gulp Missile on Cramorant-Gorging), and Taunt/Encore can force Sub users to switch out.
If you're desperate, don't be afraid to Dynamax. While Dynamaxed, opposing OHKO and phazing moves fail, and you can get some surprise KOs that way. Don't be afraid to switch out your Dynamaxed mon early if you'll lose massive momentum by not switching it out.
More set-specific tips:
Comaphazer Regieleki can be used as a blanket check to most Shell Smashers that have only boosted once (except any that outspeed at this point such as Calyrex-Shadow) by phazing them out, thus giving the Comaphazer some utility when you cannot get hazards up.
If your side has no hazards and your opponent Comaphazes you anyway, let them waste PP and phaze your Magic Bouncer in.
Assume Cramorant-Gorging always has Gulp Missile - and that it therefore is the paralysis-inducing, chip-damaging, Neutralizing Gas-ignoring, often Taunting fiend Balanced Hackmons ejected for screwing up gameplay. As a result, if your opponent has Cramorant-Gorging and it is not KOed, be very cautious about using any move that can hit Cramorant-Gorging when using any mon that hates paralysis even if the bird is currently switched out. Among other sets, this means that you have to play your own Choice Scarf non-Regieleki-non-Comatose, Taunt OHKOer, Leech Seed Taunt Trapper, and your own Cramorant-Gorging very carefully. (Shell Smashers ironically deal with Cramorant-Gorging OK by OHKOing it, although Cramorant-Gorging often has teammates that take advantage of paralyzed sweepers.)
Any lead needs to be able to deal with opposing leads, whether it's by outspeeding most of them and packing disruption and hazards such as Regieleki, restricting opposing options such as Calyrex-Shadow, or being too bulky to break in some fashion such as Eternamax. Thus, any remaining unorthodox leads need an edge (preferably over Regieleki) to remain viable. Thus, suspect any remaining Zacian-Crowned leads to have Choice Scarf and Precipice Blades (and thus outspeed and OHKO Regieleki), and treat any lead that can OHKO yours seriously (Calyrex-Shadow leads have dropped in viability due to being OHKOd or close to that by too many Knock Offs and Spectral Thiefs).
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