Recently used this team in a tour in the OM room, figured I'd share it here.
Defensive core is pretty standard stuff. Poison Mandibuzz is a super good catch-all special wall, and a great pivot against Spectrier as well. Also beats Rillaboom quite handily. Fairy Toxapex is a hard counter to almost every variant of both Rapid- and Single-Strike Urshifu, and Mandibuzz handles the Psychic variants so you're super covered against it. It's also a very good physical pivot as you can easily come in on almost any non-Ground physical attacker, get chip with Rocky Helmet and scout what they're using, then switch into an appropriate immunity/resist. Flying Heatran is the team's answer to Naganadel (and Nidoking lacking Thunderbolt), and is good at pressuring most bulky Ground and Water types as the former often pairs up with Steel and the latter is often either vulnerable to Toxic or weak to Ground. Landorus-T finishes off the core by acting as a check to mons such as Garchomp and Kartana.
The offensive core is where things start to get spicy. Fighting Choice Band Landorus-T is a super nutty breaker. Almost anything that isn't a dedicated resist is bound to get OHKO'd:
252+ Atk Choice Band Landorus-Therian Superpower vs. 0 HP / 4 Def Garchomp: 364-429 (101.9 - 120.1%) -- guaranteed OHKO
Once you get a good hit on the opponent's Ground immunity (which is often Flying Heatran, which takes a million from Superpower and outright dies to Stone Edge), you just get to spam Earthquake and watch everything drop. Most mons aren't capable of revenging you from full, some even with super-effective hits, which makes it possible to get extra kills even against offensive teams simply by trading hits. In the event that you do get hard-countered (Fairy Corviknight, for example), U-turn lets you generate momentum and bring in another mon to start breaking.
Gyarados... is there. I wanted a better way to break bulky Ground-types and didn't really have a Water/Ground switchin, and Gyarados happened to fulfill both criteria. Steel typing helps it gain opportunities to set up and sweep, and if it does get going, makes it much harder to revenge kill as Water/Flying/Steel is an absolutely stellar defensive typing and after a Dragon Dance you outspeed most of the unboosted metagame.
Dragapult is the star of the team. Steel/Dragon/Ghost is an excellent defensive typing and a very scary offensive one when paired with Fire Blast. Most teams are prepared for Specs Dragapult right now and end up being very, very weak to Dragon Dance as a result. Once Toxapex and Heatran are out of the way, getting a Dragon Dance off is basically GG.
Threatlist:
Fairy Corviknight and similar defensive mons - in theory it beats the entire offensive core, but in practice it gets overwhelmed quite quickly. Still, you have to play aggressively against it and make sure it doesn't get many opportunities to heal. Keep Stealth Rock up and force it to Defog instead of pivoting or healing, take advantage of Flying Heatran's Fighting neutrality to bring it in even against Body Press variants and pressure it out.
Kartana, Garchomp - Huge threats to the team, but you have outs against them. Don't let them set up and snowball - even if you have to trade, it's best to take them out and let your other mons deal with the aftermath.
Electric Naganadel, Tapu Koko, Fairy/Flying Zeraora, etc. - you may have noticed that 2/3 of this team is weak to Electric. Any Electric type that can consistently threaten Landorus-T is an extremely scary matchup, with Naganadel in particular being impossible to revenge kill. If you see any of these, or any other scary-looking Electric type, you need to play knowing that it's a kill-or-be-killed matchup and that you CANNOT play defense. This team has the tools to play hyper-offense, and this is what they're used for.
Dark Spectrier - probably the single worst matchup this team has. Every time it gets in, you're forced to go hard Mandibuzz and U-turn into Dragapult, risk a roll with Dragon Darts on slightly bulkier variants, and then if Dragapult goes down too early you just lose. You also can't let Dragapult get burned, and if Sticky Web is up you lose. Normal Spectrier is slightly less scary as Mandibuzz can somewhat handle it, but it's still incredibly tough to beat.
Thoughts on the meta: Naganadel needs to GO, Urshifu-SS needs to GO (Poison sucks, use Steel, Ghost or Psychic), and Spectrier needs to GO. Other than that, I don't see anything immediately overwhelming. Dragapult and Zeraora are incredibly versatile and a nightmare in the teambuilder, but I don't think either is as threatening in practice. Both easy S-tiers, but time will tell if they're actually broken. Heatran isn't as good as I thought it might be, but that might be because of how much it's tasked with answering right now thanks to how strong Tapu Lele, Naganadel and Boomburst users are - multiple of these will wear down Heatran super quickly, especially Flying Heatran with its Stealth Rock weakness. Ground Heatran and Grass Heatran are probably better than Flying, honestly: they handle Naganadel and Tapu Lele better, and have better offensive presence with either Power Herb STAB Solarbeam, or just STAB Earth Power. In general, a ton of things seem broken or insanely good on paper, but are manageable in practice due to the increased defensive counterplay from 3rd typings - Spectrier, Naganadel, Kartana and Urshifu-SS are some of the very few exceptions. Despite being manageable, however, they're almost never mediocre or bad. I nearly got my butt handed to me by Psychic/Fire DD Mew, and saw a couple other cool things either in games or on paper, like the Gyarados set posted above, Water Nidoking, and Fairy Jirachi. Lots of stuff seems super fun and in general it seems like a lot more normally "niche" or "outclassed" options could potentially thrive in this meta.
Defensive core is pretty standard stuff. Poison Mandibuzz is a super good catch-all special wall, and a great pivot against Spectrier as well. Also beats Rillaboom quite handily. Fairy Toxapex is a hard counter to almost every variant of both Rapid- and Single-Strike Urshifu, and Mandibuzz handles the Psychic variants so you're super covered against it. It's also a very good physical pivot as you can easily come in on almost any non-Ground physical attacker, get chip with Rocky Helmet and scout what they're using, then switch into an appropriate immunity/resist. Flying Heatran is the team's answer to Naganadel (and Nidoking lacking Thunderbolt), and is good at pressuring most bulky Ground and Water types as the former often pairs up with Steel and the latter is often either vulnerable to Toxic or weak to Ground. Landorus-T finishes off the core by acting as a check to mons such as Garchomp and Kartana.
The offensive core is where things start to get spicy. Fighting Choice Band Landorus-T is a super nutty breaker. Almost anything that isn't a dedicated resist is bound to get OHKO'd:
252+ Atk Choice Band Landorus-Therian Superpower vs. 0 HP / 4 Def Garchomp: 364-429 (101.9 - 120.1%) -- guaranteed OHKO
Once you get a good hit on the opponent's Ground immunity (which is often Flying Heatran, which takes a million from Superpower and outright dies to Stone Edge), you just get to spam Earthquake and watch everything drop. Most mons aren't capable of revenging you from full, some even with super-effective hits, which makes it possible to get extra kills even against offensive teams simply by trading hits. In the event that you do get hard-countered (Fairy Corviknight, for example), U-turn lets you generate momentum and bring in another mon to start breaking.
Gyarados... is there. I wanted a better way to break bulky Ground-types and didn't really have a Water/Ground switchin, and Gyarados happened to fulfill both criteria. Steel typing helps it gain opportunities to set up and sweep, and if it does get going, makes it much harder to revenge kill as Water/Flying/Steel is an absolutely stellar defensive typing and after a Dragon Dance you outspeed most of the unboosted metagame.
Dragapult is the star of the team. Steel/Dragon/Ghost is an excellent defensive typing and a very scary offensive one when paired with Fire Blast. Most teams are prepared for Specs Dragapult right now and end up being very, very weak to Dragon Dance as a result. Once Toxapex and Heatran are out of the way, getting a Dragon Dance off is basically GG.
Threatlist:
Fairy Corviknight and similar defensive mons - in theory it beats the entire offensive core, but in practice it gets overwhelmed quite quickly. Still, you have to play aggressively against it and make sure it doesn't get many opportunities to heal. Keep Stealth Rock up and force it to Defog instead of pivoting or healing, take advantage of Flying Heatran's Fighting neutrality to bring it in even against Body Press variants and pressure it out.
Kartana, Garchomp - Huge threats to the team, but you have outs against them. Don't let them set up and snowball - even if you have to trade, it's best to take them out and let your other mons deal with the aftermath.
Electric Naganadel, Tapu Koko, Fairy/Flying Zeraora, etc. - you may have noticed that 2/3 of this team is weak to Electric. Any Electric type that can consistently threaten Landorus-T is an extremely scary matchup, with Naganadel in particular being impossible to revenge kill. If you see any of these, or any other scary-looking Electric type, you need to play knowing that it's a kill-or-be-killed matchup and that you CANNOT play defense. This team has the tools to play hyper-offense, and this is what they're used for.
Dark Spectrier - probably the single worst matchup this team has. Every time it gets in, you're forced to go hard Mandibuzz and U-turn into Dragapult, risk a roll with Dragon Darts on slightly bulkier variants, and then if Dragapult goes down too early you just lose. You also can't let Dragapult get burned, and if Sticky Web is up you lose. Normal Spectrier is slightly less scary as Mandibuzz can somewhat handle it, but it's still incredibly tough to beat.
Thoughts on the meta: Naganadel needs to GO, Urshifu-SS needs to GO (Poison sucks, use Steel, Ghost or Psychic), and Spectrier needs to GO. Other than that, I don't see anything immediately overwhelming. Dragapult and Zeraora are incredibly versatile and a nightmare in the teambuilder, but I don't think either is as threatening in practice. Both easy S-tiers, but time will tell if they're actually broken. Heatran isn't as good as I thought it might be, but that might be because of how much it's tasked with answering right now thanks to how strong Tapu Lele, Naganadel and Boomburst users are - multiple of these will wear down Heatran super quickly, especially Flying Heatran with its Stealth Rock weakness. Ground Heatran and Grass Heatran are probably better than Flying, honestly: they handle Naganadel and Tapu Lele better, and have better offensive presence with either Power Herb STAB Solarbeam, or just STAB Earth Power. In general, a ton of things seem broken or insanely good on paper, but are manageable in practice due to the increased defensive counterplay from 3rd typings - Spectrier, Naganadel, Kartana and Urshifu-SS are some of the very few exceptions. Despite being manageable, however, they're almost never mediocre or bad. I nearly got my butt handed to me by Psychic/Fire DD Mew, and saw a couple other cool things either in games or on paper, like the Gyarados set posted above, Water Nidoking, and Fairy Jirachi. Lots of stuff seems super fun and in general it seems like a lot more normally "niche" or "outclassed" options could potentially thrive in this meta.