Melmetal was the first Pokémon of choice because of personal preference, I wanted to give it a try. The teammates were chosen to help with some of its weaknesses such as magnezone to trap its checks like corviknight and skarmory. Ferrothorn and Landorus-t provide hazards that wear opponents down, they can also speed it up with leech + toxic. Teammates with momentum were chosen to provide as much opportunities for melmetal to come in. While weavile can take advantage of melm weakening Pokémon like ferrothorn, or buzzwole.
Slowbro was said to be as a great melmetal partner, It provides future port but also can stand in the way of garchomp, excadrill, and urshifu-r which this team would otherwise struggle against. It forms a great core with ferrothorn as they cover many weaknesses they have. Along with intimidate support from landorus-t slowbro can be very hard to take down for opponents on the physical side. Future sight annoys the many Pokémon like toxapex, landorus-t, and skarmory which helps melmetal and weavile break through these pokemon.
Landorus-t is pretty standard with the sp. def set that helps it check many special attackers, with 25 speed IV's to under speed mirrors to get a slower u-turn. It can help wear down Pokémon with stealth rocks and toxic and forms a volt turn core with magnezone who takes advantage of pokemon like torn-t, corviknight, and skarmory which take advantage of landorus-t. It provides the team with a reliable switch in to SD garchomp, zeraora, and thunder wave hex dragapult/specs dragapult.
Weavile is the speed control of this team as otherwise the team is very slow, Jolly is needed to out speed stuff like tornaudus-t which would otherwise heavily pressure this team. It benefits from magnezone support in removing skarm, corviknight, and heavily weakening opposing melmetal. It also benefits the most from hazard stack to easily wear down Pokémon.
Ferrothorn provides a lot of defensive utility such as beating tapu koko, tapu lele, and makes urshifu-r not want to click its surging strikes which this team would otherwise struggle against. Mixed was chosen to be able to still check zeraora, melmetal, and weavile but have a better matchup into pokemon like tornadus-t, tapu lele, and dragapult. Spikes really benefit this team as it becomes really hard for its checks to repeatedly come in with hazards especially when weavile threatens to knock off leftovers and heavy-duty boots.
Magnezone forms a volt turn core with landorus-t and take advantage of Pokémon such as corviknight, skarmory and scizor checking the two main offensive threats. With specs it is also able to apply a lot of pressure in where there is no Pokémon to be trapped then abusing its high power to threaten Pokémon such as sp. def landorus-t, zapdos, and heatran.
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Some weaknesses of this team are:
this breaks through most of my team and I rely on prediction to not lose a Pokémon every single time, especially scarf which out speeds my entire team and if it claims an OHKO then it can easily snowball.
After repedatly hitting ferrothorn and melmetal with attacks it can easily snowball here as the team relies on these 2 pokemon to constantly keep it in check. While weavile has no opportunity to switch in at all since it is risking a lot such as getting knock'ed, taking a lot of chip damage, suffering a status, this has been the most managable by far.Some replays of my team working:
https://replay.pokemonshowdown.com/gen8ou-1422504235 Hail MU
https://replay.pokemonshowdown.com/gen8ou-1423037123 Kart Blace team
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I've decided to go with Tapu Koko over Magnezone, so you have some speed control on this team, furthermore Tapu Koko gives a reliable answer to Water-types such as Tapu Fini, Slowbro, Slowking, and Toxapex. Electric Terrain is a blessing here, as it powers up Melmetals Thunder Punch and makes it more threatening versus Corviknight. 
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Tornadus-T is the Defogger for this team, helping out with giving this team a reliable Kartana answer alongside answers to Tapu Bulu and Rillaboom. It can also help with Knock Off to remove items and with Regenerator it's a reliable Defogger on top. 
I only changed the EV spread so it can outpace Pokemon in the 240 Speed range, which is a blessing here. It is the Stealth Rock setter and with Toxic it can cripple Pokemon such as Zapdos, which otherwise can be annoying for this team to deal with.
Since you need a switch in to Pokemon such as Choice Specs Tapu Lele, Dragapult, and Kyurem, Assault Vest Melmetal can help here. It has its Stab move in Double Iron Bash and Earthquake as well as Ice Punch and Thunder Punch, which gives it a threatening way to be a danger to many Pokemon in the tier. 
Since your team lacks a Weavile answer besides Melmetal and Tapu Koko, I have changed Slowbro to Colbur Berry + Body Press to immediately threaten Weavile. It can also threaten Pokemon such as Ferrothorn and a plethora of other Steel-types with Body Press and immediately make a great damage output on them, which can help the entire team in the long haul. I have kept Future Sight to allow that progress can be made in the battle.