Mesprit is a very niche Pokemon, setting itself apart from other Psychic-types in the tier with its access to both Healing Wish and Stealth Rock. Unlike Uxie, Mesprit has the coverage and power it needs in order to deter Xatu from switching in, letting it set up Stealth Rock without needing Pursuit support. Mesprit also sports decent all-around bulk, letting it be a one-time check to a myriad of foes if needed. However, Mesprit's mediocre Speed leaves it vulnerable to various Pokemon such as Sneasel, Rotom, and Whimsicott, limiting its use versus more fast-paced teams.
name: Offensive Support
move 1: Stealth Rock
move 2: Psyshock
move 3: Ice Beam
move 4: Healing Wish
item: Colbur Berry
evs: 252 SpA / 4 SpD / 252 Spe
Stealth Rock is the most important move in the game and Mesprit is an effective user of the move thanks to its decent Speed and ability to threaten Xatu unlike other setters such as Steelix. Psyshock is Mesprit's best STAB move, as Psychic does significantly less damage to Cryogonal in particular. Ice Beam is Mesprit's best coverage option for threatening Xatu that attempt to switch in on it as it sets up Stealth Rock, and it also hits Pokemon such as Guzzlord. Finally, Healing Wish provides great utility in bringing a weakened teammate back to full at the cost of Mesprit. U-turn is also an option to ensure Mesprit isn't helpless versus Sneasel and is able to maintain momentum. Signal Beam has merit in hitting Pokemon such as Sneasel and Malamar.
The given EV spread maximizes Mesprit's offensive capabilities, ensuring that it does not remain passive against most teams. A Modest nature is used over a Timid nature for additional damage on certain targets such as Xatu and Whimsicott, notably OHKOing the latter with Ice Beam after Stealth Rock. However, using a Timid nature is also an option to let Mesprit Speed tie with Medicham and Gallade. Colbur Berry lets Mesprit effectively check Fighting-types such as Hariyama and Hitmonlee and also has use in allowing it to escape from Pursuit one time.
During the early stages of the game, Mesprit's aim should be to get up Stealth Rock as soon as possible. If the opposing team has Xatu, it's best to avoid the use the of Stealth Rock and instead predict it to switch in and 2HKO it with Ice Beam. The key is trying to win the mind game versus Xatu to ensure Mesprit cripples it significantly and gets up Stealth Rock successfully. When at full HP, Mesprit can also be used to switch into the likes of Hariyama, Passimian, and Hitmonlee; be wary of U-turn in Passimian's case, though. In situations where Mesprit is no longer useful or is KOed by Pursuit upon switching out, it's best to use Healing Wish in order to bring a teammate back to peak condition in order to gain some initiative.
Mesprit is most effective on more offensive teams, so setup sweepers such as Samurott, Klinklang, and Incineroar make great teammates, with each one being able to threaten some of Mesprit's checks. All of them are able to take advantage of a Sneasel locked into Pursuit and get an easy setup opportunity. Whimsicott checks most Dark-types and also breaks past Slowbro, which walls Mesprit otherwise. Decidueye also makes for a good teammate, as it threatens Slowbro and appreciates Healing Wish giving it another chance at breaking through teams. Wallbreakers such as Dhelmise and Vikavolt are typically worn down quickly, so they appreciate extra opportunities to wreak havoc on the opposing team.
A lure set using Dazzling Gleam + Fairium Z with various coverage options such as Grass Knot and Thunderbolt lets Mesprit beat conventional checks such as Guzzlord and Slowbro while also having an easier time dealing with Rhydon and Steelix. Choice Specs lets Mesprit act as a wallbreaker, setting itself apart from other wallbreakers with its access to Ice Beam and Grass-type coverage to threaten popular Stealth Rock setters such as Steelix and Piloswine. This does come with the risk of being more easily Pursuit trapped by the likes of Sneasel or turned into setup fodder for Incineroar.
Checks and Counters
**Dark-types**: Houndoom outspeeds Mesprit and doesn't fear any coverage move Mesprit is able to throw at it, letting it get a free setup opportunity if it switches in on any move besides U-turn. Sneasel can also Pursuit trap Mesprit if its Colbur Berry has been used up prior, but otherwise it is a bit shaky because it can't directly switch in due to the threat of being pivoted on with U-turn. Malamar is able to use Mesprit as setup fodder, shrugging off any attack it throws at it besides the rare Signal Beam.
**Setup Sweepers**: Incineroar and Klinklang are able to use Mesprit as setup fodder by virtue of their typing and can proceed to significantly threaten the rest of Mesprit's team.
**Ghost-types**: Faster Ghost-types such as Mismagius, Rotom, and Haunter threaten Mesprit with their super effective STAB moves, with Mismagius in particular being able to use Mesprit to set up Nasty Plot or Taunt it to prevent it from setting up Stealth Rock.