The idea behind the team is to abuse and spam psychic as much as possible with Mew and Tapu Lele. This two pokemon might seem redundant on the same team, but they serve different purposes. One of the main problems when trying to use Mew-Z or psychic terrain in general is that people will switch to psychic resists, forcing you to switch and waste turns of the terrain. With that in mind, Tapu can be used to set and reset the terrain more times than Mew can. The main problems are steel, dark and psychic types, and that is why I have tried to spread the two pokemon movepools to deal with their problems. Mew has Fire Blast for coverage and damage against steel types, and Aura Sphere to deal with Heatran and some dark types. While Tapu has Moonblast to deal with Dark types and Shadow ball for psychic types, HP fire is for emergencies to help against fire types, but for the most part, Mew is the one that deals with Steel types while Tapu handles Dark types. I also have tried Tapu with terrain extender, which has been useful, but the loss of speed has also hurt Tapu, which has helped me more as a revenge killer. A problem I found using Mew was that, without bulk investment and recovery, it can't take hits from some heavyhitters, meaning that there are less opportunities to set up. If the terrain has been already set and Mew is low in health, he could use Genesis Supernova under psychic terrain to fire a one time nuke right before it faints. You can also fire stronger Psychic attacks under psychic terrain without having to use your Z-move right away, find a good opportunity to Nasty Plot, and then use your +2 Z-move to set up Psychic terrain again and continue your killing spree. There's also the chance of pulling off a +2 Genesis Supernova under a Psychic terrain, which is usually overkill but can get you some necessary OHKO.
Just to give you an idea of how strong a +2 Genesis Supernova under Psychic Terrain is:
+2 252 SpA Mew Genesis Supernova vs. 252 HP / 252+ SpD Celesteela: 269-317 (67.5 - 79.6%) -- guaranteed 2HKO
Of course, Mew and Tapu work best being use mid to late game, when you have gotten rid of the Psychic resistances or have taken enough damage to faint from a resisted psychic under the terrain. That is why the rest of the team has been develop to aid Mew and Tapu. Phermosa is a key player of the team, being able to hit Steel, Dark and psychic types super effectively and very hard. Ice beam is for dragons and 4x weaknesses and Poison Jab is for Fairies. Her EV spread is to raise Attack with Beast Boost. Even without a +speed nature, phermosa will be outspeeding the majority of the meta, and only has to worry about a couple of scarfed Pokémon. But if they outspeed the focus sash will let her survive, hit back with a boasted attack or escape with u-turn to be used later against a slower opponent. focus Sash also helps against pokemon that have set up boosts. Also phermosa appreciates not worrying about priority moves under psychic terrain.
Landorus-T is the physical wall, mainly meant to take physical steel and dark types, lowering their attack and hurting them with rocky helmet. It also sets rocks and can use knock off to get rid of annoying scarfs or leftovers. It has a physical bulky spread, itching out of the ordinary. Lando worries about water and ice attacks, which Mantine is usually able to absorb or take well.
Mantine is my Special wall and defogger. Mantine has good synergy with the team, resisting Steel, fire, and immune to water and ground. It is helpful against Lando-t, Heatran and Scizor. Scale is the main attack move and is used to fish for burns. With Mantine's high special def and a burned opponent, it can take hits from both sides relatively well. Air slash deals with grass types and random flinches are useful to give it an extra turn of leftovers recovery. Between leftovers, roost and water absorb, it can stay healthy for a while. It worries about electric attacks, which lando and magnezone can take well. It also has a hard time with physical attackers, which lando is usually able to handle.
Finally, Magnezone helps trap Steel types like Celeestela and Ferrotorn, and can help deal with Tapu Fini. Thunderbolt is its strongest move. Flash cannon can help it deal with fairies like Tapu lele. It can also help deal somewhat against koko. Half my team carries volt/turn to help scout for good opportunities to set up Nasty Plot for Mew, or reset the terrain if necessary.