Chansey completely shapes the RBY OU metagame, and is an automatic addition to most OU teams. Thanks to its 703 HP, 308 Special, and access to Softboiled, there is no special move that can get past Chansey without taking into account stat modifiers. For reference, the most powerful special move in RBY, Articuno's Blizzard, only deals a maximum damage of 25%. This allows Chansey to sponge moves from specially based threats such as Starmie, Lapras, Alakazam, Exeggutor, Jynx, and opposing Chansey. In addition, because direct paralyzing moves such as Thunder Wave or Stun Spore mostly come from specially based Pokemon, Chansey makes the best paralysis absorber in the game, as it is potentially able to neutralize other paralyzing attempts against your team once it has been paralyzed. Moreover, paralysis protects Chansey from getting frozen while not adversely affecting its special walling capabilities.
However, while Chansey shines on the specially defensive side, its lack of STAB and mediocre, Persian-level physical bulk prevent it from being completely dominant. Specially based Explosion users, such as Exeggutor, Gengar, and Cloyster, can take out Chansey; powerful physical sweepers, such as Snorlax, Tauros, Rhydon, and Golem, can also prey on its low Defense. However, Chansey can still get past these threats with the right move. For example, it outspeeds and 2HKOes Rhydon and Golem with Ice Beam as long as it isn't paralyzed, and cripples physical attackers such as Tauros and Snorlax with Thunder Wave. Additionally, it can even run Counter or Reflect to surprise said physical sweepers.
Ice Beam is Chansey's main attacking move. Without it, Golem and Rhydon would be able to switch in for free, and Chansey would have no chance of scoring a freeze, for example, against opposing Chansey. Softboiled is a must as well, as it allows Chansey to repeatedly sponge attacks and recover HP when necessary. Keeping Chansey at high health whenever possible is recommended, as this lets it paralyze Snorlax or Tauros before having to switch out. Thunder Wave is mostly useful for crippling sweepers such as Tauros, Snorlax, Zapdos, and Persian, and often deters them from switching in. Due to the importance of paralysis in RBY, Thunder Wave is an excellent move against anything except maybe other Chansey.
For the last slot, there are two main options. Thunderbolt is the recommended move as it allows Chansey to hit Water-types such as Starmie, Lapras, Slowbro, and Cloyster for super effective damage while nabbing a fairly strong hit on Jynx. Without Thunderbolt, these Pokemon, especially Starmie, can stay in on Chansey and go for the freeze with Blizzard or Ice Beam. While Thunder Wave will still cripple them and still act as a deterrent, Chansey would be setup bait for Slowbro. While not as common, Counter is a viable alternative as well; it will often surprise Normal-type sweepers, such as Snorlax, Tauros, and Persian, that try to capitalize on Chansey's weak Defense. Due to Chansey's titanic HP, Counter will always OHKO Snorlax and Tauros using Body Slam. It should be noted, however, that some players like to check for Counter first by either using Earthquake or switching back to their own Chansey. Counter can also be used for punishing a predicted Hyper Beam. However, to do this, Chansey needs to be at full health, as a STAB Hyper Beam can deal around 75% damage.
One could also drop Thunder Wave instead of Thunderbolt in order to use Counter. However, note that Counter does not fully cover physical sweepers; Tauros and Snorlax can still use Earthquake, which, in conjunction with full paralysis or a well-timed Hyper Beam, will beat Chansey easily. Replacing Thunder Wave for Counter will also limit Chansey against Zapdos, Slowbro, and users of partial trapping moves.Team Options & Additional Comments >>>
Reflect works as an alternative to Counter and helps against physical sweepers as well. It is not as game-breaking as Counter, but it doesn't completely rely on the element of surprise and works against all physical attacks, including Earthquake from Golem and Rhydon, as opposed to Counter, which in RBY only works against Normal- and Fighting-type attacks. Reflect is especially useful against slow physical sweepers such as Snorlax, Golem, and Rhydon due to their low critical hit rate; it also helps Chansey soak up Explosion from special attackers such as Exeggutor and Gengar. Reflect Chansey will often lead to stall plays, much like Reflect Alakazam does. However, Reflect is much more common on Alakazam as it's faster and has greater freedom with its moveslots; with Reflect, Chansey has to drop Thunderbolt or Thunder Wave, which limits it against Water-types or reduces its ability to cripple sweepers, respectively.
Sing is another move Chansey can use effectively, but it's uncommon due to its poor accuracy and Thunderbolt's predominance in the last slot. This is not necessarily a bad thing, though, as Sing Chansey is unexpected and could let you score a more rewarding sleep against opposing Chansey or Snorlax, for instance. Combined with Counter, Sing makes up for a very high-risk, high-reward combination. To pull off this strategy, you need to switch a healthy Chansey into Hyper Beam from the likes of Snorlax or Tauros, put the Hyper Beam user to sleep, and then Counter whatever your opponent switches in for an OHKO. However, this strategy has many flaws that limit its effectiveness. First, Sing will miss almost half the time, and if it does, you will be left with a Chansey in the KO range of the physical sweeper's Body Slam. In addition, Chansey will have to drop both Thunderbolt and Thunder Wave, which will severely limit it. Finally, as Hyper Beam is most commonly used mid- or late-game, it's very likely that Sleep Clause will have already been activated.
While Blizzard is the preferred Ice-type move for most Pokemon, Ice Beam is recommended on Chansey, as the extra PP can make a difference in a freeze war or a stall war. Nonetheless, Blizzard's extra power can come in handy for wearing down Snorlax on the switch in a bit more quickly, as well as for nabbing the 3HKO on Tauros and the OHKO on Dragonite.
Other gimmicky options for the last slot include Light Screen, Flash, Seismic Toss, and Defense Curl; however, it's advised to stick to the options previously mentioned. The former will allow Chansey to completely wall Slowbro, while the next two help it deal with Alakazam. Defense Curl works a bit like Reflect, but has more PP and provides boosts that can be stacked. Reflect is overall the best choice for doubling Chansey's Defense in one turn, though.
Checks and Counters
Due to Chansey's limited offensive power, opposing Chansey and Alakazam can both easily neutralize her attacks thanks to Softboiled and Recover, respectively. However, these two cannot recover from a possible Ice Beam freeze. Chansey can fight back with her own Ice Beam in hopes of scoring the freeze first, but keep in mind that paralysis grants immunity to freeze, giving a paralyzed Chansey the edge in the Chansey war. Meanwhile, Alakazam can use STAB Psychic to accumulate Special drops on Chansey; in conjunction with random critical hits and full paralysis, this will eventually force Chansey out. However, Psychic will run out of PP quickly. Nevertheless, Alakazam's Recover packs 16 PP more than Chansey's Softboiled, so it's also possible for Alakazam to force Chansey out by PP stalling, but that increases Chansey's chances to freeze Alakazam if the latter isn't paralyzed. In any case, Chansey can switch back into Alakazam whenever it wants after switching out to remove Special drops or avoid wasting PP.
Chansey can also be dealt with offensively by using strong physical attacks. Snorlax is bulky enough to take some hits and can reply with a powerful STAB Body Slam or Hyper Beam, eventually beating Chansey unless it gets really lucky. If things get bad, Snorlax's Selfdestruct is powerful enough to OHKO Chansey. Rest Snorlax is especially useful when fighting Chansey, as Rest lets it shrug off paralysis. However, one will have to watch out for threatening sweepers, such as Tauros and Zapdos, switching into it while it is asleep. Rhydon and Golem can switch into Thunder Wave or Thunderbolt and outspeed a paralyzed Chansey. Rhydon's Earthquake will always 2HKO Chansey, and Golem, while not as powerful, can 2HKO with Earthquake and also has access to Explosion to OHKO in dire circumstances. These two should watch out for Ice Beam, though, as they take around 60% damage from it. Tauros and Persian hate paralysis, but they can also capitalize on Chansey's weak Defense with their powerful STAB Normal-type moves. Persian generally needs Chansey to be paralyzed to stand a chance, but if it is, Persian will usually win the match-up, even if Chansey paralyzes Persian in return. In any case, it's not recommended to switch Tauros or Persian directly into Chansey due to the threat of Thunder Wave. Zapdos and Jolteon can hit Chansey hard with Drill Peck and Double Kick, respectively. Jolteon can take hits from Chansey well, but much like Tauros and Persian, it hates paralysis. Cloyster, Dragonite, and Victreebel can potentially get past Chansey with their partial trapping moves, especially the latter two, but they rely on luck to do so. Exeggutor, Cloyster, Gengar, Snorlax, and Golem can use Explosion against Chansey; Gengar's Explosion is weaker, though, and won't OHKO a healthy Chansey.
Some physical sweeping attempts can be stopped if Chansey packs Counter or Reflect, however. Counter is especially dangerous as it might take out your Snorlax or Tauros. Reflect's ability to halve damage from physical hits will often lead to stall plays, as it can protect Chansey from physical sweepers and Explosion, but it will not make a difference against Persian, who will always score a critical hit with Slash. However, if Chansey forgoes Thunderbolt, Water-types can take it on. Starmie could neutralize Chansey and fish for a Blizzard freeze, while Slowbro can easily set up with Amnesia. Lapras and Cloyster are immune to freeze and take almost no damage from Chansey's only attacking move, Ice Beam. If Chansey decides to go without Thunder Wave instead, it would no longer be able to spread paralysis, and fast sweepers such as Tauros, Persian, Zapdos and Jolteon would now be able to take it on more easily. Moreover, the lack of Thunder Wave also limits Chansey against Slowbro and users of partial trapping moves.