Reshiram has the ability to threaten its checks on balance teams, such as Arceus-Ground, Primal Kyogre, and Arceus-Water, with Bloom Doom. Reshiram also has a solid offensive typing, making it difficult to switch into. However, Reshiram is severely hampered by its awful defensive typing, which offers it no useful resistances, renders it unable to check Fairy-types due to its neutrality to Fairy, leaves it weak to Ground-, Rock-, and Dragon-type moves, and gives it a pronounced vulnerability to all entry hazards, especially Stealth Rock. Reshiram also lies in a crowded Speed tier, which is not helped by the fact that it is forced to run a Modest nature to break through its targets; this leaves it helpless against offensive teams. Reshiram's usefulness to a team completely hinges on hitting the right Pokemon with Bloom Doom, leaving it heavily reliant on good prediction to even function. Justifying Reshiram on teams is very difficult not only due to these flaws but also because of how outclassed it is by other Pokemon in Ubers. Bulk Up Marshadow is a far superior wallbreaker due to its higher Speed, ability to boost its Attack, and extremely powerful STAB Soul-Stealing 7-Star Strike, while Kyurem-W is a better pick for Sticky Web teams due to its better offensive typing and higher Speed. Other Fire-types, such as Primal Groudon and Ho-Oh, also offer far more to teams than Reshiram through their greater power, superior defensive utility, or both. This leaves Reshiram with no real niche in Ubers.
move 1: Draco Meteor
move 2: Blue Flare
move 3: Solar Beam
move 4: Stone Edge / Roost
item: Grassium Z
evs: 4 Atk / 252 SpA / 252 Spe
Draco Meteor is Reshiram's best Dragon-type STAB move but has the drawback of lowering its Special Attack after use. Blue Flare OHKOes every Steel-type in Ubers save for Dialga and has a small chance to burn switch-ins, which can be annoying for Pokemon such as Lugia to deal with. Solar Beam allows Reshiram to use a 190-Base Power Bloom Doom once, OHKOing Arceus-Ground, Arceus-Water, Primal Kyogre, Mega Diancie, and Arceus-Rock after Stealth Rock as well as crippling Tyranitar. Stone Edge OHKOes Ho-Oh after Stealth Rock. However, Roost can be run over Stone Edge if Reshiram being totally walled by Ho-Oh is taken into account when teambuilding.
Maximum Special Attack investment with a Modest nature ensures Reshiram can hit as hard as possible and secure the OHKO with Bloom Doom on support Arceus formes and Primal Kyogre. Maximum Speed investment ensures Reshiram can at least outspeed defensive Pokemon and most base 90s that choose not to invest as many Speed EVs. Grassium Z is required to use Bloom Doom.
Reshiram's bad defensive typing makes directly switching it in difficult, with the only things it can really come in on being Ho-Oh locked into Sacred Fire, Arceus-Grass, and defensive Steel-types such as Ferrothorn. To this end, it is safest to bring Reshiram into play after a teammate has fainted, through double switches, or through U-turn support. Be careful with Bloom Doom while special walls that can easily soak it up, such as Blissey and Chansey, are alive. As Reshiram's usefulness to a team is very reliant on whether or not it actually hits its target with Bloom Doom, do not be excessively reckless with the move. It is best to try to recognize what the opponent likes to switch into Reshiram before deciding to use it. However, even this can be difficult due to how obvious it is to a skilled opponent that Reshiram is running Bloom Doom, so exercise maximum caution. Reshiram matches up poorly against offensive teams due to its poor defensive typing and bad Speed tier and struggles to beat more defensive teams that have a Blissey or Chansey; play it aggressively against offensive teams and ensure it is preserved against defensive teams until its checks are removed.
Entry hazard removal is needed for Reshiram to function properly; Excadrill and Cloyster can provide this on more offensive teams, while Arceus-Ground and Giratina-O are better picks for slightly bulkier teams. Mega Lucario, Marshadow, Mega Gyarados, and Mega Gengar can check Blissey and Chansey, which stop Reshiram cold. Priority users, such as Marshadow and Extreme Killer Arceus, can help Reshiram deal with offensive teams a little bit better. Stealth Rock users, such as Primal Groudon and Dialga, are important for allowing Reshiram to secure the OHKO on Primal Kyogre with Bloom Doom and Ho-Oh with Stone Edge. Z-Geomancy Xerneas appreciates the pressure Reshiram applies to most Steel-types. Facade Mega Salamence also benefits from Reshiram's matchup against Steel-types and ability to dispose of troublesome support Arceus formes and can provide Reshiram's team with a Ground immunity and an extra stop to all Ho-Oh variants in return. U-turn users, such as Yveltal, can assist Reshiram in coming into play more safely. Smeargle and Shuckle are interesting options as teammates, as Sticky Web heavily improves Reshiram's matchup against offense.
Dragonium Z seems appealing, as it turns Draco Meteor into a one-time all-purpose wallbreaking move in Devastating Drake. However, Devastating Drake fails to OHKO support Arceus-Ground after Stealth Rock; this leaves Reshiram very vulnerable to being OHKOed by Arceus-Ground's Judgment if it has taken Stealth Rock damage, reducing Reshiram's already limited usefulness. Tailwind lets Reshiram support its team for a few turns but is very difficult to fit. Life Orb makes Reshiram more consistently powerful but is inferior to Grassium Z, as it does not help Reshiram circumvent its checks nearly as well. Z-Sunny Day gives Reshiram a Speed boost and a boost to Blue Flare's power but is gimmicky and completely neutered by Primal Kyogre. Toxic can cripple foolhardy Arceus formes and Ho-Oh, but it is both extremely difficult to fit and useless against Primal Kyogre carrying Rest.
Checks and Counters
**Blissey and Chansey**: Blissey and Chansey are extremely specially bulky, allowing them to easily wall Reshiram and stall it out with Toxic.
**Ho-Oh**: Ho-Oh walls Reshiram that forgo Stone Edge for Roost, though switching into Reshiram is a tough ask if Stealth Rock is up.
**Faster Pokemon**: Reshiram's average Speed leaves it vulnerable to being revenge killed or heavily damaged by Pokemon common on offensive teams. Swords Dance Arceus-Ground, Deoxys-A, and Rayquaza can cleanly dispose of Reshiram without having to rely on Stealth Rock. If Reshiram has switched into Stealth Rock once, this list broadens to include Pokemon such as Marshadow, Mega Lucario, Mega Salamence, and the uncommon Palkia. None of these Pokemon can switch into Reshiram, however.
**Water-types**: Arceus-Water and Primal Kyogre can easily deal with Reshiram if it mispredicts with Bloom Doom, as they resist Blue Flare and can stand up to Draco Meteor. However, Bloom Doom OHKOes both of these Pokemon.
**Rock-types**: While none of these Pokemon can switch into Bloom Doom, Arceus-Rock, Tyranitar, and Mega Diancie can hit Reshiram super effectively with their Rock-type STAB moves and resist Blue Flare. Mega Diancie is also immune to Draco Meteor, making it even more dangerous.
**Entry Hazards**: Reshiram is particularly affected by entry hazards, as it is vulnerable to all of them and weak to Stealth Rock.
**Primal Groudon**: At full health, defensive Primal Groudon can take any one hit and OHKO Reshiram with Precipice Blades. However, a small amount of prior damage greatly reduces the efficacy of Primal Groudon as a check; for example, Reshiram only needs Primal Groudon to have lost 16% of its health before it can 2HKO it with Blue Flare and Draco Meteor. Note that Primal Groudon often has better things to be doing than checking Reshiram.