Set Description
Soft-Boiled is mandatory because Blissey tends to participate in mid- and slow-paced games and will likely get worn out over the course of a game. Blissey can usually wait to take two or three hits before using Soft-Boiled because of its amazing bulk.
The second move ensures that Dugtrio does not simply remove Blissey with zero effort. Ice Beam hits Gengar and allows Blissey to check mixed and Dragon Dance Salamence somewhat reliably. Counter is good for teams with a Pursuit trapper to get rid of Gengar or for teams that are just a little soft against physical attacks like Metagross's Meteor Mash, Swampert's Earthquake, mixed Tyranitar's Brick Break, and any of Aerodactyl's attacks. Counter should not be the primary means by which these attacks are dealt with because Blissey is usually vital to preserve for mid-game, but late-game, Blissey can lure in and take out physical or mixed attackers just as the opponent thinks a sweep is assured. Pursuit trapper + Forretress teams benefit from Counter, as well as teams that form soft-checking cores between Pokemon like Flygon, Metagross, and offensive Suicune with Zapdos or Moltres to handle physical attackers like Tyranitar and Metagross, with Superman teams—teams that use hovering Pokemon to avoid Spikes—being a prominent example of them. Ice Beam and Counter are not mutually exclusive. Traditionally, Counter, unlike Ice Beam, ensures that Dugtrio gets OHKOed regardless of its bulk. However, the emergence of Beat Up on Dugtrio, which cannot be Countered, puts forth a strong case for Ice Beam to be used alongside Counter.
By doing a fixed amount of damage regardless of the foe's defensive boosts, Seismic Toss is the most consistent attacking move because it allows Blissey to force Suicune to use Rest and beats chipped or statused Calm Mind users that have no ability to recover like offensive Celebi, Jirachi, Suicune, and Raikou. However, it still is not entirely reliable against these offensive Calm Mind users, as Blissey needs to recover quite a bit against them and can get affected by Special Defense drops or paralysis. Seismic Toss also allows Blissey to reliably chip most other Pokemon bar Gengar without concern of the incoming target's elemental resistances. Fire Blast and Thunderbolt relieve the Spikes pressure a bit by hitting Skarmory hard. Thunderbolt also clobbers Cloyster. Fire Blast 2HKOes Metagross and Heracross and OHKOes Breloom, while Thunderbolt puts immense pressure on Milotic, Suicune, Recover Starmie, and Gyarados. It also prevents Suicune from PP stalling with Rest if it is has not yet used Calm Mind, but Blissey should not be used solely to deal with Suicune in any case.
Blissey can run a variety of status moves in its filler slot. Thunder Wave paralyzes most switch-ins such as Tyranitar, Metagross, and Skarmory and helps slow Pokemon like Tyranitar, Swampert, and Metagross outrun them. Thunder Wave is also useful for stopping physical attackers in a pinch. It threatens to thwart Dragon Dance Tyranitar's and Agility Metagross's sweeps and gives Blissey an out against Aerodactyl by relying on full paralysis, all of which, while unreliable, is still notable. Thunder Wave also helps Blissey beat offensive Jirachi. Toxic is a much better status move against Tyranitar not only because it is the only way Blissey can consistently do damage to it, but also because paralysis allows Tyranitar to switch freely into Will-O-Wisp from Gengar. Toxic also forces out Swampert and Blissey and can be useful on Pursuit Tyranitar teams that do not have the physical pressure to otherwise punish opposing Blissey. Ice Beam + Fire Blast + Toxic is a good combination of moves that nails all of Blissey's common switch-ins—Metagross, Skarmory, and Tyranitar—at the expense of being beaten by Calm Mind users. Sing can completely shut down a huge threat like Tyranitar, Metagross, or Skarmory, which can help Pokemon like Jirachi and Aerodactyl. However, if it misses, it can also lure in sleep fodder like Magneton, Claydol, and Pursuit Tyranitar, all of which can be trapped by Dugtrio or one's own Pursuit Tyranitar. Sing, while unreliable, may help to take down tanks like Milotic, Rest Zapdos, and even other Blissey, denying them recovery opportunities as they get chipped down by Seismic Toss and perhaps Spikes damage.
For supportive options, Wish is good for sustaining the team, especially on more offensive Skarmory + Blissey teams. For example, it heals Choice Band Salamence by using Wish as a Fighting-type or Celebi comes in, and it also supports soft Tyranitar checks like Metagross, Flygon, and Suicune as well as soft Metagross checks like Zapdos and Moltres. Wish also supports Skarmory, especially against mixed attackers that hit it hard but can't quite get rid of it and also supports defensive Suicune, which is a worse physical wall than Swampert in sand. Wish + Counter is a common pairing on teams with a Pursuit trapper that helps to minimize the reliance on dedicated physical walls.
Heal Bell supports stall teams that use Wish or Rest on other Pokemon for longevity like Jirachi and Suicune, as well as Snorlax and Vaporeon occasionally, and also allows Claydol to use Explosion over Refresh on teams that have enough support to take down Skarmory. As an auxiliary benefit, Heal Bell also helps Blissey deal with foes that use Toxic like Swampert and Zapdos without having to switch out, possibly take Spikes damage, and perhaps let a teammate take a nasty hit or Toxic again. Note, however, that this can only be done occasionally due to Heal Bell's low PP.
The listed EVs allow Blissey to OHKO bulkless Dugtrio with Ice Beam. If special attacks are not used on Blissey, the Special Attack EVs can do into HP or Speed, and the nature can be changed to Bold. A Bold nature can still be used with Ice Beam; this especially helps Blissey against mixed Salamence's and Tyranitar's Brick Break. Unfortunately, bulkless Dugtrio is OHKOed with full certainly only after one layer of Spikes instead.
Switch Blissey into special threats or status moves. One of Blissey's biggest advantages early-game is its unrevealed moveset; one can play off the fear of firing off a status move to make a double switch to Skarmory or some other threat. Use Wish as a mid-ground move mid-game when it is unclear whether the opponent will pivot out. The key to winning games with Blissey is to exploit the free turns it gets from soaking special hits, either by forcing chip damage by itself on more defensive teams or by supporting offensive teammates with Wish, aggressive double switching, or exploiting its coverage on offensive teams such that the sequence loops in a way that is unfavorable to the opponent.
Team Options
Blissey teams tend to play a mid- to slow-paced game and want a way to punish its switch-ins continuously. Spikes is really helpful for that, and the usual suspects are Skarmory, Forretress, and Cloyster. Skarmory is perhaps the most common partner, as it lures in all the special threats, which Blissey can fend itself off against, while Calm Mind users usually don't have enough power to really hurt Skarmory before they get phazed out. Skarmory's high Defense and immunity to Spikes also allow it to switch in against any Snorlax, Metagross, and, sometimes, Fighting-types that pivot in on Blissey. Forretress also acts similarly, giving up the ability to hit Fighting-types and phaze for spinning and the ability to OHKO Magneton on the switch. Blissey can also be used with Cloyster on offensive Spikes teams, where, despite the seeming mismatch of pace, it helps to pivot against Gengar and Zapdos.
Spikes frequently comes with support aimed at getting more layers up than the opponent. Gengar spinblocks Claydol and Starmie and is great for mid- and fast-paced TSS teams, while Magneton traps opposing Skarmory and helps to enable physical attackers. Claydol and Starmie can spin away Spikes, and they can be used on TSS teams or slow-paced stall teams in conjunction with Magneton or Dugtrio. Finally, one's own Dugtrio and Magneton can trap opposing Magneton. Generally, spinning and trapping Skarmory are efforts geared towards mitigating Blissey's propensity to be Spikes fodder, while spinblocking and trapping Magneton are attempts at closing out the game by pressuring the opponent with one's own Spikes before one gets overwhelmed by the opponent's Spikes
Tyranitar is a great partner because sand really makes Blissey's bulk shine; as a comparison, while Celebi can function as a solid special wall in a vacuum, its lower special bulk becomes notable in the presence of sand and Spikes. Tyranitar suited to the pace of Blissey teams are usually the bulky physical attacking set and Pursuit Tyranitar, but there is nothing strictly restricting the use of any Tyranitar set. Tyranitar + Skarmory + Blissey is the core that is synonymous with the TSS archetype. Tyranitar also helps to make progress against Snorlax and Suicune by retaining chip damage with sand, which otherwise come in almost freely on Blissey to dish out powerful attacks or set up.
Using Blissey requires answers to physical threats like Tyranitar, Metagross, and Salamence. Bulky Water-types like Swampert, Suicune, and Milotic help Blissey deal with them, and their passivity tends to invite Electric-types in, which Blissey can deal with. Suicune isn't too robust given it lacks the Rock-type resistance of Swampert and the immediate recovery of Milotic, so Wish or Aromatherapy support is a great option to go with Suicune. Another answer not to be missed is Dugtrio, which cannot switch into Tyranitar and Metagross but can trap them with some chip damage. Dugtrio also threatens to stop pesky Celebi from harassing Blissey with Leech Seed and Jirachi from setting up on Blissey.
Offensive Pokemon with good pivoting abilities are great partners for Wish Blissey. Choice Band Salamence can come in on Fighting-types and Celebi. Metagross, Flygon, and Suicune can pivot in against Tyranitar. Moltres, Zapdos, and different Suicune variants can pivot in against Metagross. Most of these Pokemon are frail physical checks in a vacuum, but they become a lot more reliable when supported by Wish.
Finally, bulky setup sweepers like Curse Snorlax and Wish Jirachi also appreciate Blissey's ability to support them with Wish or Aromatherapy, respectively, and they also appreciate Blissey's special walling presence mid-game so that their HP can be preserved for late-game.