your team has nice some nice type variety, but a lot of mixed attackers, which you wouldn't want. I haven't done help with teams yet, so forgive me if i'm a bit rash or if i'm not doing this right.
Serperior
My favorite pokemon, glad you're using it! Leaf Storm + Contrary is obvious on her, and Giga Drain is a nice touch. There are a few things I'd change though. First of all, get rid of the Meadow Plate and use something better. Leftovers is common on her, with substitute as well. here's an example Serperior you can try out;;
Serperior @ Leftovers
Ability: Contrary
EVs: 4 HP / 252 SpA / 252 Spe
Modest Nature
- Leaf Storm
- Substitute
- Hidden Power [Rock]
- Leech Seed
Serperior has decent bulk and is fast, so instead of Timid, Modest would be better to boost her SpA more. Setting up substitute can do wonders, getting recovery from leftovers and getting to take a free hit. hp rock will cover 4/5 weaknesses. serperior is a bit too frail for synthesis, so replacing it with another move would be good. Leech seed can be switched out for things such as glare or taunt, or even aromatherapy, since your team doesn't have any heal bellers/aromatherapy-ers.
Mimikyu
The famous pikachu of the alola region!! Fairy and ghost is a great advantage, with disguise, even better. The way you run it is,,, questionable. Mimikyu is too frail for pain split, and will 9 times out of 10 get bullet punched or something when it's on 1hp. The Focus Sash is a good idea, along with Jolly nature. kudos friend! If you want, here's a common example of how most people run Mimikyu;;
Mimikyu @ Focus Sash
Ability: Disguise
EVs: 4 HP / 252 Atk / 252 Spe
Jolly Nature
- Play Rough
- Shadow Claw
- Shadow Sneak
- Swords Dance
Mimikyu gets a STAB priority move, shadow sneak, which is VERY useful or finish others off. Shadow Claw and Play Rough are it's main ways of attacking, and swords dance makes use of it's Disguise ability. Easily set up and destroy most of the opposing team. pretty simple on running it.
Tapu Koko
Tapu Koko has a nice typing, getting rid of it's fairy type weakness to steel, which is common. Tapu Koko has higher attack than special attack, so I recommend using a physical set. I've never used Tapu Koko myself, but looking at it's stats and move-pool, I can give an idea of how I would use it on your team;;
Tapu Koko @ Life Orb
Ability: Electric Surge
EVs: 4 HP / 252 Atk / 252 Spe
Adamant Nature
- Brave Bird
- Thunder Wave
- Wild Charge
- U-turn
Brave bird is one of his most powerful moves, with Adamant and Life Orb boosting his attack to insane levels. Wild Charge is respectively his most powerful STAB move. I want to warn you though, both cause recoil, so replacing the moves or the item would be understandable. If you don't like recoil, Acrobatics with a Flying Gem could replace Brave Bird, and Spark could replace Wild Charge, though I recommend keeping Wild Charge. U-Turn is for that get away and Thunder Wave is to cripple foes. If you choose the set above, consider changing Thunder Wave out for roost for reliable recovery. Your team already has a fairy type, even though Tapu Koko has no Fairy moves in this set, it still off balances your team. Changing out Tapu Koko for another electric type would be good. Consider Alolan-Raichu or Raiku, with Special Attacking sets respectively.
Tyranitar-Mega
Tyranitar has incredible attack when it mega evolves, and has nice physical bulk too. Be careful of it's 4x fighting weakness though. Ice Punch and Flamethrower aren't the best options, which should be replaced. Stone Edge and Crunch are good moves though. Here's an example move-set I whipped up for Tyranitar;;
Tyranitar-Mega @ Tyranitarite
Ability: Sand Stream
EVs: 252 HP / 252 Atk / 4 SpD
Adamant Nature
- Crunch
- Dragon Dance
- Earthquake
- Iron Head
Tyranitar is a mean machine, which if brought in correctly, can force Pokemon to switch out, giving a chance for a free D-Dance. Earthquake is a good move, but you can choose to replace it with Stone Edge or Rock Slide. Iron head will cover your fairies, and Crunch is for good ol STAB. when using him make sure to go against pokemon in your favor. Say a stray Infernape came by and obviously had Mach Punch on it. You'd switch, don't try to e-quake. Make sure you're in a situation in your favor. Tyranitar has a lot of weaknesses, so you'd better be prepared to switch out into a Pokemon that withstands them.
Ash-Greninja
He has incredible Special Attack, with good speed. The large downfall of using him though, is how long it takes to activate his Battle Bond Ability. Have another pokemon in your party weaken a pokemon first, then when you know you can safely switch in, go for the priority water shuriken. That's the easiest way I can think of activating the ability. I honestly dislike Ash-Greninja, but without a doubt it can be used to cause havoc in a battle. Getting rid of Shadow Sneak is a good idea, since you already have water shuriken. replacing it with something else that covers your fighting weakness is a good idea. Surf can be replaced with Ice Beam or Scald, since you already have a water move, I'd go with Ice Beam. Here's an example set you could try out;;
Greninja-Ash @ Focus Sash
Ability: Battle Bond
EVs: 4 HP / 252 SpA / 252 Spe
Timid Nature
- Ice Beam
- Water Shuriken
- Dark Pulse
- Extrasensory
Not much to say here, Ice Beam and Extrasensory are for your weaknesses, and the other moves are stab and are boosted greatly when Battle Bond is activated. Personally, I'd change Greninja out for another water type. Tentacruel, Feraligator, and Starmie are good options. Politoad too.
Celesteela
Last but not least, the flying steel thing. Love it's name, but the design is weird in my opinion. A nice type with a nice ability and nice move-pool. nice. Beast Boost raises his Special Attack, which is good for the set that you wanted to run. I agree with the majority of moves you chose, but changing them out isn't too bad. Getting rid of E-Quake is a start, since Celesteela is a special attacker, not physical. Keeping Air Slash and Flash Cannon is A-Okay, which is actually what I did in my example move-set. Fire Blast has great, but doesn't have a lot of PP and low accuracy. Switching it out for another fire move is a good idea,, in my opinion. With out further a do,, here's what I would run;;
Celesteela @ Leftovers
Ability: Beast Boost
EVs: 252 HP / 252 SpA / 4 SpD
Modest Nature
- Air Slash
- Flash Cannon
- Giga Drain
- Flamethrower
Leftovers brings the best recovery next to Giga Drain for Celesteela. Modest nature boosts his special attack, and the moves are near self explanatory. I changed out Fire Blast for Flamethrower after some debating, since it has higher accuracy and more PP, which means more time out on the field for Celesteela. Beast Boost can dangerously let his special attack rise, to the point where it's mostly a OHKO. Personally, I'd keep Celesteela in your team. It's pretty good.
Overview
Your team is sort of balanced, all of them are mixed attackers, which needs to fixed. You also need a tank / wall to join the party, giving your team more reliability. Having a wish user would also be handy, that way it can support it's teammates with recovery since the majority of them are all out attackers. You have a few pokemon that share a type, which can lead to problems. Make sure you have counters to moves that threaten a pokemon on your team, say Serperior might get hit with Flare Blitz. You have a counter, Tyranitar, to switch into.
//this is long im sorry//
///probs did this wrong LOL//
