Moves
Most of the time Chansey inherits the moves of the Pokemon it transforms into, so the selected moves are chosen to help Chansey deal with the rare instances in which Imposter fails to activate. Shore Up allows Chansey to recover its HP, meaning it can still operate as a bootleg defensive Pokemon should Imposter not activate. Whirlwind is used to phaze the foe should they be behind a Substitute, as that disables Imposter from activating. It also serves as a way to handle setup sweepers, as Pokemon such as Primal Kyogre and Mega Gengar can very easily set up on Chansey and overpower it. U-turn lets Chansey pivot out safely to a more threatening teammate to avoid being taken advantage of. Metal Burst provides Chansey with some offensive presence. As it deals damage based on the amount of HP Chansey takes from a move that turn, it can inflict some serious damage if used correctly. Once shown, it can also discourage the opponent from attacking Chansey in fear of being KOed in return. Final Gambit is an option to essentially net a trade KO. This could be useful if KOing a switch-in is more beneficial than keeping Chansey around. Thanks to Chansey's sky-high HP stat, so long as it is at a moderately high amount of HP, it can KO a large majority of the metagame. Trick can be used to cripple an enemy with a burdening item. It's recommended to only use Trick when equipped with a Choice Scarf, as giving up Eviolite is seldom is a worthwhile trade-off. Alternative move options include Parting Shot over U-turn to net some momentum against Pokemon Chansey has failed to Transform into. It also comes with the added benefit of ignoring Substitute, one method of preventing Imposter from activating, while also forcing Magic Bounce Pokemon to switch out, though this could easily backfire as it'd give the opponent a free switch to whatever they desire. Refresh could also be chosen for its ability to remove any burdening status ailments, as they carry over even when transformed. Lastly, Spectral Thief or Haze removes any stat boosts the foe may have, somewhat alleviating Chansey's big weakness to being made setup fodder. These could be chosen over Whirlwind, if one forgoes that move, in case Chansey's team struggles to check specific sets that Whirlwind is useless against such as a sweeper with Magic Bounce.
Set Details
Eviolite boosts Chansey's defenses both prior to and during transformation, granting Chansey a lot of additional bulk and enabling it to effortlessly deal with so many Pokemon one-on-one. A viable alternative to Eviolite is Choice Scarf. Although it gives up a lot of bulk and locks Chansey into one move, it'll be able to outspeed setup sweepers rather than simply Speed tie with them. A Relaxed nature is used to further boost Chansey's lackluster Defense stat whilst minimizing its Speed stat. This is important, as moves like Metal Burst and U-turn are more effective when used last. However, sometimes 252 Speed EVs with an Impish nature can be beneficial, as then Chansey will outspeed opposing Imposter users and Transform into them if both Imposters are switched in at the same time. This allows you to safely scout the opposing Chansey's set as well as use their moves without losing PP.
Usage Tips
Always keep in mind that simply bringing a Chansey to a battle puts immense pressure on the opponent, as they must utilize their offensive Pokemon carefully to avoid being swept by their own Pokemon. Another important note is that Chansey is incredibly weak before Transforming, as it cannot defend itself very well against Pokemon that can switch in and damage it heavily or use certain moves like Spore. Therefore, one should actively prevent Chansey from participating in battle untransformed as much as possible. In most instances, Chansey is an extremely effective lead, as it can safely scout its foe's set without being in danger of being trapped and PP stalled. Only in certain situations (for instance, when leading off against a self-Imposterproofed Mega Gengar) will it be a poor choice. Even in this instance, though, you can still determine your best course of action safely after scouting the opposing set's moves. Pivot Chansey in whenever possible to scout for the foe's set. That information could prove incredibly useful when making decisions later on in the battle. It is encouraged that one switch Chansey in on a defensive Pokemon like Giratina or Mega Audino, as in most instances, they can do very little damage to it. If it does fall low on HP, however, one can switch it in on said defensive Pokemon and utilize their own status moves or abilities such as Regenerator or Magic Bounce with little resistance, as often those Pokemon can do little to stop Chansey from doing so. One might also wish to double switch Chansey in when predicting an opponent's switch, as that is a good way to determine a Pokemon's set. It should be noted, however, that certain Pokemon such as Zygarde-C, Registeel, and Mega Steelix may run sets specifically designed to trap and KO Imposter users with moves like Anchor Shot, hoping to catch Imposter users and PP stall them until they're KOed. For this reason, Chansey should be careful not to recklessly switch into Pokemon that commonly use these sets. Despite having a high HP stat and access to Eviolite, Chansey can ill afford to switch into hard-hitting threats repeatedly, especially if the aforementioned threats are they themselves frail, as you'll be copying their defensive stats as well, or hit super effectively by their own moves. Examples of frail threats include Deoxys-A and Mega Sceptile, and examples of threats hit hard by their own moves include Red Orb Groudon and certain Mega Gengar sets. However, if they are also boosting their defensive stats through the means of Quiver Dance, Chansey can actually switch in quite safely. Thanks to Imposter's mechanics, Chansey is effective at stalling PP, as it can refresh its limited PP simply by switching out. This gives Chansey the edge in PP stalling situations.
Team Options
Due to the nature of Imposter, Chansey can fit on nearly every team archetype and is very self-sufficient, so it rarely requires team support to function. Having said that, Chansey appreciates being able to switch in freely, which slow allies such as Registeel, Aegislash, and Mega Audino can allow it to with U-turn or Parting Shot. The aforementioned Pokemon are also useful for setting up entry hazards to not only deter Sturdy Shedinja, a Pokemon Chansey has a tough time beating, from switching in but also deal some chip damage to assist in a potential countersweep. It also breaks any Focus Sashes that frail Pokemon like Deoxys-A commonly use to protect themselves against Imposter Chansey, as they are frail yet powerful enough to OHKO a Transformed Chansey after setting up. As Chansey takes damage from hazards before transforming, it's susceptible to every form of hazard, which can wear it down rather quickly. The aforementioned slow allies all do well as hazard removers. Pokemon that can handle common Judgment users like Mega Gengar and Arceus make for great teammates, as Chansey cannot handle them in a one-on-one situation. These Pokemon include Yveltal and Primal Kyogre, which are valuable for their ability to check Mega Gengar, easily the most common user of this strategy, with ease. Another archetype of sweepers Imposter Chansey struggles to beat one-on-one are Poison Heal sweepers. Users like Regigigas, Xerneas, and Primal Kyogre often have no problem putting Chansey to sleep and then slowly beating it down, taking advantage of the Imposter's lack of Poison Heal recovery. Teammates like Zygarde-C and Giratina are some tried-and-true checks to Regigigas, whereas some Primal Kyogre sets and Toxic Orb Magic Guard Ho-Oh can deal with Xerneas handily. Lastly, Primal Kyogre is checked by Red Orb Groudon and RegenVest Primal Kyogre. RegenVest users also are helpful for Chansey, as not only can they deal with the problematic Mega Gengar, they also switch into opposing Imposter Chansey and take advantage of its passiveness to recover HP. Viable RegenVest Pokemon include Yveltal, Primal Kyogre, and Dialga. Teammates with Misty Surge are also beneficial, as they help protect Chansey from crippling status ailments. Chansey's status carries over every time it is Transformed, meaning if it is burned it cannot be as effective as a physical damage dealer, for instance. Defensive pivots like Giratina and Primal Kyogre are some examples of viable Misty Surge users.
