Sun Battle Tree Team

[Star Champion AJ]

This is the team I'll be using for the Battle Tree in Pokémon Sun.

Vikavolt@Quick Claw
Ability: Levitate
Nature: Modest
EVs: 252HP/252 SpAtk/4Spd
Bug Buzz
Thunderbolt
Energy Ball
Flash Cannon
The first member of the team whose gimmick is using Quick Claw's effect to outspeed the opponent sometimes (20% as long as the opponent doesn't use something with a higher priority like Sucker Punch or Protect).
Bug Buzz and Thunderbolt are the STAB moves of choice, with Energy Ball and Flash Cannon providing coverage.
The Nature and EV spread maximizes Vikavolt's damage output.
Offensively, Vikavolt covers Flygon's Fairy weakness with Flash Cannon and Primarina's Grass weakness with Bug Buzz.
Defensively, Vikavolt's Fire and Rock weaknesses are covered by both Flygon and Primarina.

Flygon@Muscle Band
Ability: Levitate
Nature: Jolly
EVs: 252Atk/4SpDef/252Spd
Earthquake
Dragon Claw
Crunch
Rock Slide
The second team member and my favourite Pokémon.
Earthquake and Dragon Claw are the STAB moves, Crunch and Rock Slide are the coverage moves.
The Nature and EV spread maximizes Flygon's Speed and the Muscle Band gives an extra oomph to Flygon's physical attacks (20%), but I may change to Life Orb (30% at the cost of some HP) or a Choice Band (50% but locks Flygon into using only one move) after getting more knowledge of the Battle Tree's opponents.
Offensively, Flygon covers Vikavolt's weaknesses with Earthquake (Fire and Rock) and Rock Slide (Fire) and Primarina's Electric and Poison weaknesses with Earthquake.
Defensively, Flygon's Fairy and Ice weaknesses are covered by Vikavolt's Flash Cannon and the Dragon is covered by Primarina's Moonblast and Ice Beam. Also, Flygon can deal with some Dragon types like Goodra or Drampa but has difficult with the bad boys like Salamence or Garchomp.

Primarina@Shell Bell
Ability: Torrent
Nature: Modest
EVs: 4HP/252SPAtk/252Spd
Moonblast
Scald
Ice Beam
Psychic
The last member of the group and the Alolan starter.
Moonblast is the Fairy STAB while Scald is the Water STAB for utility reasons (30% chance to burn non-Fire types). Ice Beam covers Grass types and Psychic deals with Poison types.
The Nature and EV spread increases Primarina's damage output and the item restores Primarina's HP after attacking.
Offensively, Scald covers Vikavolt's weaknesses and Moonblast deals with the bad boys Flygon struggles.
Defensively, Primarina's Grass weakness is covered by Vikavolt's Bug Buzz and Primarina's Electric and Poison are covered by Flygon's Earthquake.

Also, since Vikavolt and Flygon are both immune to Ground, Cynthia's Garchomp would be a nice partner in Multi Battles so she can spam Earthquake while I take no damage from her attacks.

That's the team folks! If you have any opinions about items, moves and maybe EVs, feel free to give your feedback.

 

[Sceptile Leaf Blade]

You've got a good team in terms of type coverage, but I'm missing ways to amplify your damage output. You don't have any status moves on any of your pokémon, and none of your items boost your damage output either. Defensive pokémon with recovery are going to be very tough to break through, especially the likes of Blissey and Regice. Flygon is not a bad pokémon by any means, but when you don't do anything to amplify its damage, it simply won't deal sufficient damage to win battles. You need to run boosting items like a Life Orb and possibly a move like Dragon Dance to get into good KO ranges. Vikavolt can work because it has a very high Sp.Atk stat, but it's usually wise to run some kind of damage boosting on it as well. Personally I think Vikavolt is more suited to Trick Room teams than regular teams. Primarina is a bit slow but can be powerful, personally I like to run Sparkling Aria instead on it with Primarinium Z, but Scald also works. Do note that a 30% rate to burn is far from reliable and can't be used as a main deterrent to shield your physical defence.

One thing about King's Rock, it's usually not worth it. 10% rate to flinch statistically means a 10% boost to your damage, and only if you are actually faster than your opponent, and only if you're not running Rock Slide since the two don't stack. An item like a Z-crystal, a Choice Band, or a Life Orb usually yields far more benefit. Or run something like a Sitrus Berry with Dragon Dance setup to get a bit more reliability.

 

[Star Champion AJ]

You don't have any status moves on any of your pokémon...

Well, I made a test run before posting this team and I saw a pretty high amount of trainers using Taunt on their first opportunity when I'm going for offensive moves anyway. In one battle, the opponent used that move twice.

You've got a good team in terms of type coverage...

Well, I don't have Fire (Escavalier is quite common in Battle Tree) or Fighting (Exploud is a common offender) type moves to exploit weaknesses effectively. The only Pokémon among the three that can use those moves (Flygon) has a nature that lowers special attacks (Fire Punch is only available in USUM) and Superpower ruins Flygon's attack and defense (an non-issue with Choice items that favor hit and run tactics) and can only be learnt as a Trapinch.

Defensive pokémon with recovery are going to be very tough to break through, especially the likes of Blissey and Regice.

Yep, I tend to struggle a bit with defensive walls, even with Vikavolt's massive Special Attack, but I can 1-2HKO some of them and 3 hits may be enough to remove most of them, assuming Double Team is not used.

Flygon is not a bad pokémon by any means, but when you don't do anything to amplify its damage, it simply won't deal sufficient damage to win battles. You need to run boosting items like a Life Orb and possibly a move like Dragon Dance to get into good KO ranges.

One thing about King's Rock, it's usually not worth it. 10% rate to flinch statistically means a 10% boost to your damage, and only if you are actually faster than your opponent, and only if you're not running Rock Slide since the two don't stack. An item like a Z-crystal, a Choice Band, or a Life Orb usually yields far more benefit. Or run something like a Sitrus Berry with Dragon Dance setup to get a bit more reliability.

About items for Flygon, I kinda of agree that there are better items than King's Rock. However, while the Choice Band and Life Orb have the best boosts, both have drawbacks.
I'm not a big fan of the Choice Items because while they boost a certain stat by 50%, they lock you into using only one move until you switch or the Pokémon goes down and Flygon has two moves that are useless in certain situations (Earthquake doesn't work on Flying-types and Levitate Pokémon and Dragon Claw is blocked by Fairy-types and the AI is smart enough to switch when it has a Pokémon that is immune to either move) and I need the freedom to switch moves when needed.
About the Life Orb, it increases the damage output by 30% but also lowers Flygon's survivability as well.
So if I had to use an item that increases damage, I'd go with Muscle Band (20% for physical attacks), but the boost may not be enough for certain threats.
About Z-Crystals, I only used them twice in game (once against Ilima and against Lusamine's Clefable), but I believe they could come in handy. If I went for that route, a Groundium-Z, a Dragonium-Z (maybe with Dragon Rush) or a Rockium Z (with Stone Edge) would be great, but I'd lose the utility effect of an attack boosting item.
So I'll test with Muscle Band for the time being and after a few battles I may switch to Life Orb. I'll only use the Choice Band (or a Z-Crystal) when and only when I get some more knowledge about the Battle Tree's opponents.

And thanks for the feedback.