
Fireceus is a great mon, but what makes it truly great is it not being passive while simultaneously threatening FC walls. Tidy Up is severely underrated solely for its ability to remove enemy Substitutes. At +1 Attack, Arceus can get 2HKOs or OHKOs on a shockingly high amount of the meta. The main problem you will encounter using it is Imposter as you can not risk Fireceus being Poisoned.
Alternate Options: Heavy-Duty Boots can be used if you find Stealth Rock to be an issue, but I generally don't and can typically handle hazard removal fine without it. Not being Paralyzed or haxed is far more valuable to you. Flame Plate provides a modest buff to V-create plus the immunity to Trick. Tera Ghost is an alternate viable Tera Type that allows for a different generic defensive typing that lacks the Steel-type weakness in trade for a Ghost- and Dark-type one.
Improof: Eternatus, Slaking in a pinch.

Eternatus is a genuinely great mon hindered by a lack of particularly great sets. Unfortunately, its typing does it no favors either. But Base 130 Speed and a huge amount of bulk with great Special Attack allow it to remain a constant threat. This set is more or less a theorymon that I made to find viable SetPedia sets, but this paid off rather handsomely. Mold Breaker allows it to be very, very versatile. Especially with Toxic, as Toxic gets --- accuracy with Poison-types while Eternatus can reliably improof Arceus.
- Good as Gold, Magic Bounce, and Purifying Salt are vulnerable to Toxic.
- Strength Sap bypasses Good as Gold, Magic Bounce, and... maybe Liquid Ooze.
- Torch Song bypasses Well-Baked Body, which is notable as WBB Zacian-C typically counters Eternatus.
- Both Torch Song and Dragon Energy bypass Ice Scales.
However, it finds itself annoyed by RegenVest Goodra-Hisui and Dialga-Origin due to it having trouble with beating them without the use of Terastal, but the option is there should the need arise. Imposter is a bigger worry due to Eternatus having a natural Poison immunity and it likes to try and force out Eternatus to prevent it from snowballing.
Note: Never go to +2 Special Attack on Eternatus while the opponent has an active Chansey or Blissey. Your improof risks getting overrun.
Alternate Options: Covert Cloak can be used on Eternatus, however Leftovers (Black Sludge stops working on Terastallized Pokemon) synergizes better with Dragon Energy. Moveset should not be changed or Eternatus loses significant effectiveness. Tera Type can be Fairy if you want a defensive Tera Type rather than an offensive one.
Improof: Blissey, Ting-Lu in a pinch.

It's Imposter. Use it to scout, use it to win against certain offensive matchups, but most importantly use it to PP stall if you cannot reasonably win through offensive tactics. As for the moveset itself, it is solely for improofing Eternatus and nothing else. Amnesia is there to allow Blissey to avoid getting overrun by Torch Song. Jungle Healing cures Toxic. Low Attack means enemy Imposter doesn't heal at all. Salt Cure is there to have passive damage against both Imposter and any switch-ins.
Alternate Options: Do not use Choice Scarf on this set. Blissey needs Covert Cloak. But Eviolite Chansey is
acceptable here. The major change I'd say is viable here is Ruination or Comeuppance over Salt Cure if you're worried about an enemy Covert Cloak Blissey. Haze and other means of removing Torch Song boosts are likely unviable as they cause Blissey to be too heavily pressured especially as Blissey always underspeeds Eternatus. It is possible you could run Heal Bell over Jungle Healing to better support the party at the cost of a slightly dodgier improof.
Improof: N/A

Ting-Lu is here over Dialga or Goodra as the team would otherwise be too weak to Ground-type attacks. It does its job fine as Knock and Salt Cure is very hard to wall on the same set and it readily takes on most special attackers besides Flutter Mane and Enamorus. It's not a particularly inspired set, but it does its job and it does it well for this team, providing an essential Nuzzle and Prankster immunity. Ting-Lu should prioritize Knocking Off items at any point, and Salt Cure any switch-ins. Salt Cure as mentioned is also great for harassing defensive sets that otherwise get in the way of the rest of your party.
Alternate Options: Ability Shield is usable if you're super afraid of Normalize Entrainment, but Toxic on Eternatus or Arceus can still hit Flutter Mane and enemy Arceus through Normalize. Tera Type should not be changed, and while the moveset can be changed, I do not particularly have any suggestions. Both Ting-Lu and Dondozo can change up their moves if you desire, however they work perfectly fine as is.
Improof: Knock Off the enemy Imposter's item and Salt Cure them to force them out. Eternatus can improof in an emergency as Toxic severely threatens enemy Imposter.

Slaking is the late game cleaner, but he can work as a breaker in some cases too. However you should be skittish with it especially given that it does
not like the presence of Imposter and needs to wait for an opportune time to come in. Magic Guard provides some extremely useful immunities with hazards, status, and Salt Cure. It also is immune to Rocky Helmet. Unfortunately, due to the status immunity, this means a Poisoned Imposter isn't going to be threatened coming in unless you Terastallize. Rock Tera Type is generally an ass type, but the Fire resistance is very useful in some non-Imposter matchups and it resists Normal too.
As an important note, while it seems counterintuitive, you should focus on removing hazards with Arceus over Slaking as your team is significantly handicapped if Slaking even risks being crippled or knocked out. Slaking is best used only after you know what exactly your opponent has in a given matchup. Or if Imposter is down, then scout to see what mon your opponent brings in on Slaking once you set up with it.
Alternate Options: Wide Lens is an obvious one, but Life Orb gives more power that's less consistent. This LO power has won me games though so I still use it personally. Morning Sun can be used in favor of Sap, but Slaking generally wants the PP. Victory Dance is usable, however Slaking generally appreciates being able to remove hazards and Substitute.
Improof: Terastal, PP stalling with Strength Sap if needed. Parting Shot spam can work in an emergency. Just because Slaking
can self-improof, doesn't mean it should, and you should try to get Imposter down over time or at the very least put it in a disadvantageous position so it thinks twice before coming on in. Slaking generally always wants to come in late game or if the need to break something is absolutely necessary to avoid dealing with Imposter directly. Bringing Slaking in on mons without pivot moves gives Imposter less breathing room especially if it risks coming in as +0 on you Tidying Up.
Slaking is genuinely not hard to use despite the precarious improof. Imposter doesn't get full accuracy Population Bomb and its rather easy to annoy out with Eternatus and Dondozo unless you get extremely unlucky. That being said, some opponents may only use Imposter to PP stall you and this is another reason to avoid bringing out Slaking early until you have a gameplan which results in it getting at least one KO before switching out. Slaking is almost always going to put in work even despite this Imposter problem especially as it tanks almost everything extremely well and can force out certain mons with Strength Sap before setting up.

Dondozo is a fairly typical Prankster. It's likely you've already used this thing before given its bulk and the necessity to have a fat physical mon on your teams. This Dondozo is rather passive but Ceaseless Edge makes it extremely annoying to deal with as the opponent needs to focus on bringing out hazard removal which can give you a momentum advantage, especially if their removal doesn't like one of the other party members. Prank Parting Shot is very valuable in this metagame as Dondozo does not particularly mind Ting-Lu switch-ins and this team is not
super afraid of Hoopa-U. Dondozo should mostly be used reactively, but if the game stalls at all you can start aggressively setting up Ceaseless Edge to try and force the aforementioned momentum advantage.
Alternate Options: Rocky Helmet is an option, but not for improofing Slaking as its immune to Rocky Helmet damage via Magic Guard. Only use Rocky Helmet if you think its worthwhile. Morning Sun is an option over Shore Up, but I specifically chose Shore Up to avoid Rain reducing Dondozo's healing. Tera Type is not too important, use Ground or whatever type you feel is necessary; Dondozo is unlikely to change any matchups as a result of Terastal.
Improof: Arceus removes hazards and forces Imposter out at the threat of Toxic. Ting-Lu prevents Parting Shot from letting Imposter gain momentum and the threat of 1/4HP Salt Cure + Knock Off forces Imposter out as well. Slaking is usable but not advised as an improof.