Mega Starmie 
Water/Psychic
Huge Power
60 HP / 100 Atk / 105 Def / 130 SpA / 105 SpD / 120 Spe
Main STAB options: Psychic, Psycho Cut, Zen Headbutt, Aqua Jet, Flip Turn, Hydro Pump, Liquidation
Useful Moves: Agillity, Bulk Up, Dazzling Gleam, Grass Knot, Ice Beam, Ice Spinner, Rapid Spin, Recover, Scald, Thunderbolt
Pros:
- Very high speed stat, especially in comparison to the limited dex allows it to get the jump on many crucial foes.
- Huge power in tandem with 100 base attack makes it nigh unwallable on the physical side.
- It's high special attack allows it to successfully pull off mixed sets, using its coverage to threaten foes that would otherwise check its physical sets, such as Corviknight.
- It's movepool allows for man different sets, from sweeper with Bulk Up to Pivot with Flip Turn to Utility piece with Rapid Spin and Recover.
Cons:
- It's low HP stat makes it relatively frail, limiting its switch in opportunities.
- It's quite vulnerable to faster foes or priority, namely Dragapult and Kingambit both of which hit it super effectively with Ghost- and Dark-type STAB respectively.
- It suffers from 4 move slot syndrome, struggling to fit in all the tools it wants into one set and often lacking the tools to break common checks.
- Its STAB options on the physical side are rather middling power wise, not giving it the extra bump in breaking that it needs verse some threats like Corviknight.
Prospective Sets:
Sweeper (Starmie) @ Starminite
Ability: Huge Power
EVs: 252 Atk / 4 SpD / 252 Spe
Jolly Nature
- Bulk Up
- Liquidation
- Zen Headbutt
- Aqua Jet / Ice Spinner / Rapid Spin
This set is likely to be one of the most common sets for Starmie, allowing for it to become a potent physical sweeper, reaching truly ridiculous levels of damage output with its STABs. The last move allows for the most versatility in the set, Aqua Jet provides strong priority to hit potentially faster foes, Ice Spinner gives Starmie Coverage for the Grass- and Dragon-types that resist Liquidation, while Rapid Spin provides a Speed boost to amplify its sweeping potential while giving the set some team utility.
All Out Attacker (Starmie) @ Starminite
Ability: Huge Power
EVs: 252 Atk / 4 SpA / 252 Spe
Naive Nature
- Liquidation
- Zen Headbutt
- Rapid Spin / Flip Turn / Recover
- Thunderbolt / Ice Spinner / Flip Turn
This set aims to hit very hard while providing some utility in pivot or hazard control. Flip Turn deals very sizeable damage to switch ins while safely getting a partner in, being a phenomenal option for Starmie. Thunderbolt hits Corviknight, Ice Spinner hits Grass- and Dragon-types. Recover can be used to increase its longevity, taking advantage of the many switches it forces to heal up. This set can also excel in cleaning weakened teams.
Teammates:
- Corviknight and other bulky pivots can switch into foes that attempt to revenge KO Starmie, especially Grass-types, and allows Starmie safe switch ins with a slow U-Turn.
- Partners that share common checks such as Corviknight pair well with Starmie, as they can break the defensive cores for each other, allowing the other to effectively sweep/clean.
- Starmie loves entry hazards to chip its checks on switch in, allowing for it to break through them with more ease.
Checks:
- physical walls such as these can be difficult for starmie to break if it lacks the coverage / team support.
- offensive foes that can outspeed (some with the help of priority) and revenge Starmie are particularly problematic, especially if they can safely switch into one of more of its STAB attacks.
- certain foes such as Mega Gyarados can alleviate their bulk and typing to use Mega Starmie as set up opportunity.
Water/Psychic
Huge Power
60 HP / 100 Atk / 105 Def / 130 SpA / 105 SpD / 120 Spe
Main STAB options: Psychic, Psycho Cut, Zen Headbutt, Aqua Jet, Flip Turn, Hydro Pump, Liquidation
Useful Moves: Agillity, Bulk Up, Dazzling Gleam, Grass Knot, Ice Beam, Ice Spinner, Rapid Spin, Recover, Scald, Thunderbolt
Pros:
- Very high speed stat, especially in comparison to the limited dex allows it to get the jump on many crucial foes.
- Huge power in tandem with 100 base attack makes it nigh unwallable on the physical side.
- It's high special attack allows it to successfully pull off mixed sets, using its coverage to threaten foes that would otherwise check its physical sets, such as Corviknight.
- It's movepool allows for man different sets, from sweeper with Bulk Up to Pivot with Flip Turn to Utility piece with Rapid Spin and Recover.
Cons:
- It's low HP stat makes it relatively frail, limiting its switch in opportunities.
- It's quite vulnerable to faster foes or priority, namely Dragapult and Kingambit both of which hit it super effectively with Ghost- and Dark-type STAB respectively.
- It suffers from 4 move slot syndrome, struggling to fit in all the tools it wants into one set and often lacking the tools to break common checks.
- Its STAB options on the physical side are rather middling power wise, not giving it the extra bump in breaking that it needs verse some threats like Corviknight.
Prospective Sets:
Sweeper (Starmie) @ Starminite
Ability: Huge Power
EVs: 252 Atk / 4 SpD / 252 Spe
Jolly Nature
- Bulk Up
- Liquidation
- Zen Headbutt
- Aqua Jet / Ice Spinner / Rapid Spin
This set is likely to be one of the most common sets for Starmie, allowing for it to become a potent physical sweeper, reaching truly ridiculous levels of damage output with its STABs. The last move allows for the most versatility in the set, Aqua Jet provides strong priority to hit potentially faster foes, Ice Spinner gives Starmie Coverage for the Grass- and Dragon-types that resist Liquidation, while Rapid Spin provides a Speed boost to amplify its sweeping potential while giving the set some team utility.
All Out Attacker (Starmie) @ Starminite
Ability: Huge Power
EVs: 252 Atk / 4 SpA / 252 Spe
Naive Nature
- Liquidation
- Zen Headbutt
- Rapid Spin / Flip Turn / Recover
- Thunderbolt / Ice Spinner / Flip Turn
This set aims to hit very hard while providing some utility in pivot or hazard control. Flip Turn deals very sizeable damage to switch ins while safely getting a partner in, being a phenomenal option for Starmie. Thunderbolt hits Corviknight, Ice Spinner hits Grass- and Dragon-types. Recover can be used to increase its longevity, taking advantage of the many switches it forces to heal up. This set can also excel in cleaning weakened teams.
Teammates:
- Corviknight and other bulky pivots can switch into foes that attempt to revenge KO Starmie, especially Grass-types, and allows Starmie safe switch ins with a slow U-Turn. - Partners that share common checks such as Corviknight pair well with Starmie, as they can break the defensive cores for each other, allowing the other to effectively sweep/clean. - Starmie loves entry hazards to chip its checks on switch in, allowing for it to break through them with more ease.Checks:
- physical walls such as these can be difficult for starmie to break if it lacks the coverage / team support. - offensive foes that can outspeed (some with the help of priority) and revenge Starmie are particularly problematic, especially if they can safely switch into one of more of its STAB attacks. - certain foes such as Mega Gyarados can alleviate their bulk and typing to use Mega Starmie as set up opportunity.