Weaknesses: Fighting (2x)
Resistances: None
Immunities: Ghost

Strategy:

This is the standard set for Blissey that has essentially remained unchanged since the 3rd generation. Her main role is to defend against special attacks and she certainly does an excellent job of doing that. With massive HP, excellent Special Defense, and the ability to learn Softboiled, Blissey is nearly impenetrable against special moves.

Her second job is to heal your team of any status ailments by using Aromatherapy, which could be a lifesaver for those unfortunate moments when your main sweepers get statused. But as useful as Aromatherapy may be, it gives your opponent a free turn to switch in a counter or set up. Therefore, you can use Thunder Wave as an alternative move to cripple any sweepers on the switch-in.

On the offensive side, Blissey's main move is Seismic Toss which is her only way of dealing consistent damage. Blissey also has a large arsenal of special moves and the most useful of these is Ice Beam since it covers a wide variety of types. Ice Beam is also important for taking on Garchomp, who can come in and Swords Dance in your face if you lack an Ice move.

Despite being a special wall, Blissey already has such high HP and Special Defense that she doesn't need any SDef EVs at all. Instead, you'll always want to make Blissey's Defense as high as possible by giving her a Bold Nature and 252 Defense EVs. After that, you can split the rest between HP and Special Attack. Generally, you'll want to put enough EVs in HP so that Blissey's total HP is divisible by 16. By doing this, you'll maximize the healing efficiency of her Leftovers (which restores 1/16th of your health every turn). Good numbers to aim for are 688 (which take 148 EVs) or 704 (which take 212 EVs). After HP is done, place the rest of your EVs into Special Attack. And finally there's 252 HP / 252 Def / 4 SAtk if you just want to be super defensive, although it's not entirely recommended since you won't have a chance of OHKOing Salamence and other Dragons with Ice Beam.

Wish Blissey

Other options: Counter, Grass Knot, Flamethrower, Charm, Stealth Rock, Sing, Toxic, Snatch,

Counter can be an unexpected surprise against opponents that try to target Blissey's weak Defense. It is also the most effective way for Blissey to defeat Dugtrio, one of her major counters. Be careful when using it though. A wrong prediction with Counter could ruin the surprise factor and lower its effectiveness for the rest of the battle.

For special attacks, Grass Knot is useful for defeating Swampert, Tyranitar, and Rhyperior - three Pokemon that Blissey often has trouble with. Don't expect an OHKO though. Flamethrower can be a surprise for Metagross and Heracross, but again, it won't OHKO them.

Blissey has a wide variety of support moves that she can also use. Charm is a new addition that Blissey only gets from Happiny. It can seriously annoy physical attackers, but you cannot have Charm and Seismic Toss on the same set. Stealth Rock can be useful since Blissey causes a lot of switching among special sweepers although it is advised that you put Stealth Rock on another Pokemon since Blissey doesn't have the room to use it.
Sing and Toxic are handy status moves, but not quite as useful as Thunder Wave. Sing can disable a potential threat, although it has terrible accuracy. Toxic fits Blissey's ability to stall and can be an annoyance to many opponents. Snatch is an underrated move that lets you steal some beneficial moves from the opponent, including Substitutes and a wide variety of stat boosts. Many opponents like to set up against Blissey and Snatch discourages them from doing so.

How to counter Blissey:
You may think that taking down Blissey with physical hits would be easy due to its low defense. However, that's not the case. Blissey can make up for a bad Defense stat by having the highest HP in the game so that even some physical moves can have a hard time defeating her.

Blissey can't stand STABed Fighting attacks so any Fighting-type using a move like Close Combat, Cross Chop, or Focus Punch has a good chance of OHKOing her. The main thing to watch out for is Thunder Wave. If Blissey paralyzes your Fighter, it can greatly lower its ability to attack. One way around this is to bring a Fighter with the Guts ability (Heracross, Machamp, or Hariyama) to take the Thunder Wave and therefore activate Guts to increase its Attack power. Heracross is particularly threatening since its advantage over other Fighting types is that it can use Pursuit to hit Blissey for some major damage as she switches out.

Dugtrio is one of Blissey's nemesis since it can switch in on a Thunder Wave and 2HKO her with a Choice Banded Earthquake. However, it gets taken down easily with Ice Beam or Counter, so you need to take caution when switching it in.

Other non-Fighting physical attackers that can make short work of Blissey include Snorlax, who doesn't mind getting paralyzed since it's already slow. Rhyperior is immune to Thunder Wave, doesn't mind Ice Beam due to its Solid Rock ability, and packs a powerful STABed Earthquake and Stone Edge. Electivire can take advantage of Thunder Wave to boost its speed and hit Blissey hard with Cross Chop.

Special attackers should not plan on defeating Blissey any time soon. Not even Focus Blast is enough for a 2HKO. Porygon-Z gets special note because it can OHKO Blissey with Hyper Beam after a Nasty Plot boost (which shows how powerful it can be). Grass types with Leech Seed can enjoy the fact that a seeded Blissey restores the user's team of quite a lot of HP every turn while she can't do much in return (though you'll need a sturdy Grass type that wouldn't fall easily to Ice Beam).

Pokemon that can reach 404 HP can make 101-HP Substitutes. This makes their Substitutes unbreakable from one Seismic Toss and also prevents them from getting statused. Meanwhile, they can safely stat boost in Blissey's face without having to worry about taking damage or getting paralyzed (this applies to both physical and special sweepers).

Damage Calculations:

Defending - Special
350 SAtk Salamence's Draco Meteor + Choice Specs vs. 714 HP/306 SDef Blissey: 36-43%
405 SAtk Alakazam's Focus Blast + Choice Specs vs. 714 HP/306 SDef Blissey: 48-56%
405 SAtk Porygon-Z's Tri-Attack + Life Orb + Adaptability vs. 714 HP/306 SDef Blissey: 31-37%
405 SAtk Porygon-Z's Hyper Beam + Life Orb + Adaptability vs. 714 HP/306 SDef Blissey: 58-59%
383 SAtk Azelf's Psychic + Life Orb + Nasty Plot vs. 714 HP/306 SDef Blissey: 44-52%
349 SAtk Azelf's Psychic + Life Orb + Nasty Plot vs. 714 HP/306 SDef Blissey: 40-47%

Defending - Physical
259 Atk Dugtrio's Earthquake + Life Orb vs. 714 HP/130 Def Blissey: 39-46%
259 Atk Dugtrio's Earthquake + Choice Band vs. 714 HP/130 Def Blissey: 45-53%
284 Atk Dugtrio's Earthquake + Life Orb vs. 714 HP/130 Def Blissey: 43-50%
284 Atk Dugtrio's Earthquake + Choice Band vs. 714 HP/130 Def Blissey: 50-58%

233 Atk Focus Punch (no STAB) vs. 714 HP/130 Def Blissey: 54-64% (guaranteed 2HKO)
221 Atk Explosion vs. 714 HP/130 Def Blissey: 85-100%
260 Atk Explosion vs. 714 HP/130 Def Blissey: 100% (guaranteed OHKO)

Links to Pokedex Entries

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