Reshiram is a mandatory story encounter exclusive to Pokémon Black Version. It can only be found within N's Castle and comes at Level 50.
With a STAB combination completely unresisted as a result of its exclusive ability, Reshiram is the definition of an offensive powerhouse, with a titanic base 150 special attack to vaporize those who stand before it. The problem is that it's stuck with four moves until the postgame due to the events following its capture and Dragonbreath is not going to be enough against the array of Pokémon that both N and Ghetsis bring to the table in order to confine its rampage.
N (N's Castle): While Zekrom is two levels higher than Reshiram, N thought getting rid of Dragonbreath was a good idea, evening the playing field a bit. However be careful as Giga Impact has a noticeable chance to OHKO in contrast to Dragonbreath's 2HKO. Regardless, if he starts spamming Fusion Bolt, you win as it is a 4HKO, just keep healing and be ready to switch because of the elephant in the room, Light Screen, which swings this fight massively in his favour. In regards to the rest of his team, Archeops stomps on Reshiram, with Stone Edge being a likely OHKO and it is noticeably faster. Somehow Carracosta is an easier target as it shares a 2HKO with Reshiram with your dragon effortlessly outspeeding. The threat of critical hits does mean that is probably best suited for a Pokémon that is not weak to Rock however. Klinklang and Vanilluxe both are incinerated with ease and Zoroark only lasts slightly longer.
Ghetsis (N's Castle): Cofagrius will probably irritate Reshiram through its Toxic stalling battle plans more than deal actual damage, being 2HKO'd by Fusion Flare. Similarly Bouffalant has a mutual chance to 2HKO between Fusion Flare and Head Charge, but it is significantly slower. Bisharp suffers the same fate as Klinklang while Eelektross is 2HKO'd in contrast with Acrobatics' 3HKO. Note that is 7 shots of a 5PP move, you are not sweeping him with this, outside of item support. Said dream is shot the moment Seismitoad or Hydreigon come out to deal with you. Seismitoad will take 2 Dragonbreaths before 2HKOing with Earthquake and Hydreigon has Dragon Pulse and a higher speed stat to kill you with.
Post-Game: You actually have access to its TM movepool atop its move relearner stuff now. Additionally, its an incredibly powerful legendary Pokémon up against overleveled unevolved Pokémon for the most part, go wild.
When caught, Reshiram will have the following moves: Dragonbreath, Slash, Extrasensory and Fusion Flare. Hopefully you are content with this, as you will not have a chance to change it before the postgame. Speaking of which the first move Reshiram picks up is Dragon Pulse at level 54, which should immediately replace Dragonbreath. Level 64 brings Imprison, which is pointless. Crunch at level 71 might have a use but there is probably little left to deal with besides Cynthia at this point. Beyond this is Fire Blast(level 78), Outrage (85), Hyper Voice (92) and finally, its other signature move, Blue Flare at level 100.
While its move relearner options are redundant with what it starts with, outside of maybe Ancientpower, the vast white Pokémon has a decent selection of TM options, albeit only accessible in the postgame. Psychic serves as a solid upgrade over Extrasensory, letting Reshiram hammer Fighting and Poison-type Pokémon. Shadow Ball serves the same purpose for Ghost and Psychic-types. Focus Blast is redundant with Fusion Flare outside of Normal and Rock-types. Solarbeam and Sunny Day lets Reshiram decimate Water-types (and everything else through the boost to Fire-type moves). Stone Edge/Rock Slide serve as a way to hit Fire-types, although Rock Slide is weaker than Dragon Pulse in most situations. Lastly, Flame Charge is worth mentioning simply due to boosting its decent base 90 speed. Lastly, through tutor, Draco Meteor is a nuke option, decimating nearly everything that lacks a resistance to Dragon, although the harsh special attack drop may be unappealing.
Recommended Moveset: Fusion Flare, Dragon Pulse, Psychic/Shadow Ball/Focus Blast/Solarbeam, Flame Charge/Sunny Day/Stone Edge/additional move from the 3rd slot.
Actual Moveset pre-postgame: Fusion Flare, Dragonbreath, Slash, Extrasensory
Water-types: Reshiram struggles to deal with the Rock-types that N has to offer, which these Pokémon can take on without issue. They also serve as a soft check to hard answer to Hydreigon to boot. In response Reshiram disintegrates any Grass-type that looks at them funny in the postgame as well as taking out Ghetsis' Eelektross.
Grass-types: These murder Carracosta significantly harder than Reshiram can, avoiding the risk of a critical Stone Edge in the process. They also handle Seismitoad for it, leaving it free to burn up most of the Pokémon that get in its new allies way (Besides the Hydreigon)
Ground-types: In the entirely possible event that Zekrom forces Reshiram to withdraw, it is useful to have something that can smack it through its Electric-typing to get it out of the picture quickly. Reshiram can serve as an defroster against Vanilluxe and unmask Zoroark, both threats against the best Ground-types the region offers.
Psychic-types: Ironically a type weak to Dark is surprisingly well fitted to engage Ghetsis' Hydreigon due to it lacking a STAB move to hit them Super effectively with. While their ability to do significant amounts of damage back is varied, they can all set up on it. Sadly no Reuniclus though, it is in the other version. In exchange Reshiram turns the Dark and Ghost-types actually able to harm them into ashes.
Minimum stats are calculated with 0 EVs, IVs of 0, and a hindering nature, if applicable.
Maximum stats are calculated with 252 EVs, IVs of 31, and a helpful nature, if applicable.
What Nature do I want? Modest without a shadow of a doubt, because more dakka is always good.
How good is Reshiram in a Nuzlocke? It is simultaneously broken and underwhelming. As weird as that concept may sound, it is an incredible Pokémon. With high stats across the board and a godly offensive typing atop its decent coverage options, Reshiram can obliterate anything it faces... in the postgame. It is only available for the final two battles in the game, both of which carry several Pokémon that provide trouble for Reshiram. To make matters worse, it is locked to its starting moveset until the postgame, which only compounds these issues. If it lives to see Eastern Unova, it will burn everything in its path, and that should inspire you to utilise care with it in the final story battles.
Weaknesses: Ground, Rock and Dragon
Resistances: Bug, Steel, Fire (x1/4), Grass (x1/4) and Electric
Neutralities: Normal, Fighting, Flying, Poison, Ghost, Water, Psychic, Ice and Dark