rate my gen-7 ou team for showdown

Question

mimi-tans (Mimikyu) @ Life Orb
Ability: Disguise
Shiny: Yes
EVs: 252 Def / 252 SpA
Careful Nature
- Swords Dance
- Dazzling Gleam
- Shadow Sneak
- Wood Hammer

Charizard-Mega-X @ Charizardite X
Ability: Tough Claws
Shiny: Yes
EVs: 4 HP / 252 Atk / 252 Spe
Adamant Nature
- Dragon Dance
- Earthquake
- Fire Punch
- Will-O-Wisp

Sableye @ Expert Belt
Ability: Stall
Shiny: Yes
EVs: 248 HP / 128 Def / 132 SpD
Careful Nature
IVs: 0 Atk
- Substitute
- Toxic
- Recover
- Protect

Alakazam @ Life Orb
Ability: Magic Guard
Shiny: Yes
EVs: 250 Atk / 252 SpA / 6 SpD
Adamant Nature
IVs: 0 Atk
- Energy Ball
- Psyshock
- Shadow Ball
- Hidden Power [Fire]

Garchomp @ Poisonium Z
Ability: Sand Veil
Shiny: Yes
EVs: 252 Def / 4 SpA / 252 SpD
Impish Nature
- Earthquake
- Rock Slide
- Poison Jab
- Swords Dance

Greninja @ Expert Belt
Ability: Protean
EVs: 8 HP / 252 Atk / 248 SpD
Naughty Nature
IVs: 0 Atk
- Dark Pulse
- Ice Beam
- Water Shuriken
- Grass Knot

here is the poke-paste. https://pokepast.es/e1bef84d953603ff

Answer 1

• I would recommend giving mimikyu an adamant or jolly nature to boost its sweeping capabilities. Replace dazzling gleam with a STAB physical move like shadow claw or play rough. Also, i would get rid of wood hammer because mimikyu is already frail enough with life orb recoil and no recovery options.
• sableye is only really viable in its mega evolution form. Replace substitute with knock off or another support move. you can replace toxic with willowisp for the burn but it's up to you. If for whatever reason you want to keep sableye unevolved, replace expert belt with another more useful item like leftovers or rocky helmet.
• alakazam almost always runs a timid nature. give 252 evs in both speed and special attack (alakazam is a special attacker not a physical attacker).
• greninja should have a timid or modest nature. If you're going for a wallbreaker 4 attack set you should run choice specs. evs should be 252 in special attack and speed.
• mega charizard has relatively low base speed compared to a lot of other pokemon in ou so i would recommend a jolly nature. willowisp could be replaced with roost as a recovery option or dragon claw for a strong stab move that helps against opposing dragon types. flare blitz could be replaced with fire punch if you don't mind the recoil.

• your garchomp set needs work. try using this site for some ideas "https://www.smogon.com/dex/sm/pokemon/garchomp/"

• the main problem with this team overall is it's mostly made up of glass cannon setup sweepers and wallbreakers. It's also missing a hazard remover which is important considering your charizard has a 4x weakness to rock and will lose half of its health to stealth rock. I would consider replacing your garchomp with a hazard setter/remover such as excadrill. Here's a sample moveset:

Excadrill @ Leftovers/Focus sash
Ability: Mold breaker
EVs: 252 Atk / 4 Def / 252 Spe
Jolly Nature
- Earthquake
- Iron Head
- Stealth Rock
- Rapid Spin

• you could also replace grass knot with spikes on your greninja which is viable even on a choice specs set.

have a look at the commonly used roles on this site to give you some ideas: "https://www.smogon.com/dp/articles/pokemon_dictionary"
for example, a screen setter could be used to help your pokemon set up more safely or a support pokemon to remove status conditions like burn and paralysis which would otherwise cripple your pokemon.

Good luck :)