Just like Blissey in OU, with great Special Defense, enormous HP, and instant recovery in Softboiled, Chansey remains the dominant special wall in UU. Access to both Wish and Aromatherapy to keep her teammates healthy also makes Chansey a very important member on stall teams. Unfortunately, Chansey's utility comes to a halt after that; holding the title of the lowest Defense stat in the game, she will crumble to almost any physical attack. Seismic Toss is also Chansey's only viable option of dealing direct damage, which means she is easily shut down by Taunt users, Ghost-types, RestTalkers, and setup sweepers. Even with these glaring issues, Chansey still consistently fulfills her job as a fantastic team player and full stop to most special attackers, cementing her position as one of the most frustrating defensive threats to face.
The way this set plays is virtually identical to Blissey in OU, except with weaker defenses and the inability to use special attacks effectively. However, Chansey still has phenomenal special bulk, which allows her to switch into almost every special attacker with ease. She can then proceed to use Wish and Protect to heal herself or pass the Wish to replenish the health of a teammate. Seismic Toss is Chansey's only worthwhile offensive option; its consistent 100 damage will often outdo even super effective special attacks, and despite it not deterring switch-ins too often, the damage eventually racks up.
Chansey is almost forced to use a status move in the last slot, but it is by no means just filler. Toxic is the move of choice for stall teams, where Chansey should be played. It hits Flying-types and Levitators who are immune to Spikes and Toxic Spikes and works nicely alongside Wish and Protect to stall out opponents. Toxic also cripples Mismagius and Rotom on the switch, who can otherwise render Chansey useless with Substitute as they are immune to Seismic Toss. Thunder Wave is generally more useful on teams that don't rely heavily on residual damage, since it cripples offensive threats for another team member to pick off. A Calm nature is preferred to handle powerful special attackers, like Nasty Plot Houndoom, better; Bold is an alternative for less vulnerability to strong physical attacks.Team Options & Additional Comments >>>
Like Blissey, Chansey is a natural choice for a cleric. She can safely switch into a majority of special attackers and rid your team of status while the opposing Pokemon switches out. Chansey's ability to switch in so easily also makes her an ideal user of Stealth Rock, as she can often set it up early in the match; its high PP also compensates for Aromatherapy's low PP, thereby allowing her to compete in PP stall wars. Softboiled is the recovery move of choice since it heals instantly and only takes up one moveslot, unlike Wish + Protect, allowing Aromatherapy to fit in Chansey's moveset. Seismic Toss once again functions as Chansey's only viable option to deal direct damage.
If your team can fit Stealth Rock elsewhere, Chansey can opt for a status move instead. The choice of move depends on your team and how exactly you want Chansey to threaten opponents. This set in particular needs to be somewhat threatening in order to discourage powerful sweepers from switching in, lest their key Pokemon gets crippled. Thunder Wave is the most reliable, generic option; most teams appreciate paralysis off of 100% accuracy. Toxic is best for stall-based teams that focus on gradually wearing down the opposition. Bold can be used over Calm to take some physical hits better, but Calm is often better in situations against boosting special sweepers such as Nasty Plot Houndoom.Team Options & Additional Comments >>>
Heal Bell is identical to Aromatherapy, except it fails to heal Pokemon on your team who have the Soundproof ability. It does have one advantage, though: it is legal with Wish, unlike Aromatherapy. This means Chansey can support her team as both a Wish passer and a cleric on the same set, but another move must be sacrificed. Softboiled can also be used with Wish, but it suffers from the same problem: adopting it requires Chansey to forfeit an important moveslot.
Special attacks such as Flamethrower and Ice Beam can theoretically be used by Chansey; however, Chansey's base 35 Special Attack stat is so low that Seismic Toss will do more damage in nearly every situation. The exceptions are against Ghost-types, although Shadow Ball doesn't even 2HKO any of them, and some frail Pokemon with quadruple weaknesses, who are not threatening enough for Chansey to specialize against. Calm Mind doesn't rectify this problem either, as even at +6 Special Attack, Chansey fails to 2HKO any special wall unless it's with a super effective attack. Clefable outclasses Chansey as a Calm Mind sweeper in most aspects due to her higher Defense and Special Attack and is a better option for using special attacks if you dislike Chansey being restricted to Seismic Toss.
Shed Shell can be used instead of Leftovers to avoid being trapped by Dugtrio. Charm is a neat option to cripple physical attackers on the switch, but it is illegal with Seismic Toss and Wish. Since few neutral physical hits actually OHKO Chansey, Counter is a good option to surprise opponents, although it is risky against Pokemon who may use a setup move like Swords Dance or Dragon Dance. Light Screen may sound like an odd choice considering Chansey's sky-high Special Defense stat, but she gets many opportunities to support her team with it.
Checks and Counters
This is where Chansey's lack of offensive prowess comes back to bite her. Generally, physical attackers with strong STAB moves will force Chansey to quit stalling and get out of play. Hariyama is easily Chansey's number one counter; his gigantic 144 base HP lets him easily take Seismic Toss, Guts versions appreciate status, and he can OHKO her with Close Combat. Other Fighting-types, including Blaziken, Toxicroak, Hitmonlee, and Hitmontop, can also OHKO Chansey with powerful Fighting-type attacks.
Behind Fighting-types are powerful physical attackers who don't care about status. Nidoking is immune to both Thunder Wave and Toxic and can force Chansey out with the threat of Superpower. Thunder Wave variants of Chansey are setup fodder for Torterra and Rhyperior, while Toxic variants cannot do much to Aggron, Drapion, and Venusaur. Drapion and Absol can also threaten Chansey with Choice Band Pursuit, though both must be wary of Thunder Wave. Dugtrio can trap and OHKO Chansey with Beat Up, assuming the majority of its teammates have reasonable base Attack stats and are in good condition. Dugtrio can also 2HKO Chansey with Earthquake, although Life Orb variants can be stalled out with Wish and Protect. Azumarill can set up 101 HP Substitutes on Chansey for a free Focus Punch or just flat out destroy her with Superpower.
Some special attackers can break through Chansey with stat boosting or by sheer force. Mismagius must avoid Toxic on the switch, but if she's behind a Substitute, Chansey cannot touch her, merely giving her free turns to accumulate Calm Mind or Nasty Plot boosts. At +6 Special Attack, Hidden Power Fighting is a 2HKO, so PP stalling isn't an option for Chansey either. Similarly, Calm Mind + RestTalk Spiritomb and Calm Mind Clefable don't care about Toxic, so they can use Chansey as pure setup fodder.