First, almost all your Pokémons are unviable and all their items are bad. I’m going to do lots of sections of this answer so I’ll be making a lot of posts.
If it weren’t for Mewtwo, this team should have been in UU, but is more of a RU team, maybe even ZU
Your main team
First of all, this team isn’t very good. This first post doesn’t change any of your Pokémons which you prefer.
Mewtwo
Your set isn’t the best, Psystrike is okay. I would put Life Orb on Mewtwo instead, Thunderbolt instead of Aura Sprere. Although Recover is good, I feel that Mewtwo isn’t bulky so if you don’t care about accuracy(like me even after missing 90 accuracy moves four times in a row!) use Focus Blast, else, Aura Sphere. Weird EV spread and I would run Timid nature on Mewtwo as its SpA will be boosted by Nasty Plot. It’s EVs should be full SpA and Spe if you want to hit hard, and also 4 Hp EVs. And don’t forget 0 Atk IVs! Fire Blast is also an option to cover Bug types. Focus Sash is also an option
Mew: TOTAL TAUNT BAIT!
Why Focus Sash? Mew is often used defensively. At least one of its moves are okay. Change Focus Sash to Leftovers or Eject Button, replace Transform with Stealth Rocks, a hazard setter will be useful. Change Roost to Taunt, that way your set ups won’t be defogged. Also, your opponents will only attack you and if you run Red Card, you get switched out. Also change Scald to Spikes for more set up. Run Max Def and SpD EVs and also 4 HP EVs.Sorry if I am addicted to making total taunt bait sets.
Metagross
This isn’t really bulky, but I’ll make it one while also being offensive Although Leftovers is the standard defensive mon item, is better offensively so I’ll give it the item Assault Vest. All your moves need to be changed! So I’ll straight away tell you them.
Moves: Bullet Punch, Meteor Mash, Earthquake and Body Press.
It’s recommended nature is Adamant but Impish is fine. You should run full EVs on Atk and Def while running 4 EVs on SpD.
Lapras
Lapras isn’t offensive! And you should hold Leftovers, due to the Item Clause, Mew has to hold a Red Card.
Since you got all of its moves wrong, I’ll do the same thing I did to Metagross
Moves:Roar, Toxic, Reflect and Heal Bell
I would run calm nature, making the two defensive stats almost equal with the EV spread. The EV spread would be running Max HP EVs, 228 Def and 28 SpD EVs.
Sylveon
Since you are using Light Screen, mind as well replace Calm Mind and use Reflect to set up dual screens, also making more use of its Light Clay item. Why Pixilate? You get nothing from it as you have no normal moves so use Cute Charm instead. WHY SPE EVS ON SYLVEON? What you should do is run this:
EVs: 252 HP / 128 Def / 4 SpA / 124 SpD
As for your last Pokémon, I thought it would be Calyrex-Shadow, but....
It is Yveltal!
I saw that all your members are very weak to Calyrex-Shadow and decided that Yveltal would be the best. Life Orb would be a good start. Oblivion Wing will heal back Yveltal’s health, Dark Pulse is STAB and Dark Aura boosted. Heat Wave can check Necrozma-DM and Zacian-C. Sucker Punch is the move that saves your team from Calyrex. The nature I would recommend would be Hasty. Run Max EVs for SpA and Spe and run 4 EVs on Atk.
This is how it should look like in the end: Your improved team
I’ve hoped I’ve helped!
