Battle Moveset

A Good Moveset for Blissey would have to be this:

Attacks:

Seismic Toss
Thunder Wave / Ice Beam
Aromatherapy
Softboiled

Items Attached:

Leftovers

Preferred Trait:

Natural Cure

Preferred Nature:

Bold

Strategy Using Blissey

Classic Blissey, bane of new players and a staple of OU play. While people have advanced enough to prepare themselves against Blissey it can still fair greatly in OU. With it's huge HP stat and Special Defence stat as well as Softboiled it makes for a Special Wall that'll drain it's opponent of PP sooner than going down and being one of the few 'good' Pokémon with access to Aromatherapy/Heal Bell it provides it's team with a great source of support.

Seismic Toss is mainly a filler move, it's a guaranteed source of damage and against any opponent with no form of self-recovery it'll eventually beat them down. With Thunder Wave it can bring sweepers to a halt, since a more common counter to Blissey is simply to sling a physical sweeper out against it this can do your team plenty of favours, against the faster ones especially. Ice Beam on the other hand serves a handful of purposes: Firstly, it means Gengar can't set up a Substitute and then proceed to Focus Punch Blissey. Secondly, it hits several common Pokémon very strongly, Salamence and Dugtrio being the ones who get the worst of it. Thirdly, it's a source of damage that isn't static, if an opponent has pitiful Special Defence then you may be able to do more damage with Ice Beam than Seismic Toss, it should also be noted that unlike Seismic Toss this move can hit Ghosts to, tying in with the aforementioned Gengar purpose.

For Blissey to be best used it needs to be teamed up with appropriate Pokémon, it needs a solid physical wall (or two preferably) to take the physical hits that it attracts and it needs Pokémon to counter it's common counters. In most cases when you use Blissey you'll be best off using a stall team alongside it, a team mainly made out of tanks who win the battle out of survival rather than force, it's a team like this that Blissey will excel in.

EV Corner:

EVs: 252 HP / 252 Def / 4 SDef
Bold Nature (+Def, -Atk)

There's no two ways to go about it, Max Defence is a necessity on Blissey. With it's huge HP stat it can take a couple of physical hits with it's Defence maxed, without max Defence it can potentially cave to a single physical attack. On the other hand with Max Special Defence you'll see very little difference, it'll still suck up those Special hits just as easily without any Special Defence boost. The only variation to this EV set-up is slightly less HP (212 EVs to maintain Max leftover recovery) in favour of Special Attack.

Other Optional Sets

Blissey @ Leftovers
Trait: Natural Cure / Serene Grace
EVs: 252 HP / 4 SAtk / 252 SDef
Calm Nature (+SDef, -Atk)
- Ice Beam
- Psych Up
- Thunderbolt / Thunder
- Softboiled / Wish

Psych Up Blissey, the Blissey for Ubers play. It can Psych Up opposing Uber's Calm Minds and then out-stall them, possibly getting the better of them. Max Special Defence is favoured in this situation since Uber's is one of the few situations where it'll make a difference. This is one of the few sets where Serene Grace can come of use, with Thunder you have a 60% chance of getting paralysis and in a stalling battle you have a fairly high chance of landing a lucky freeze with Ice Beam.

Blissey @ Leftovers
Trait: Natural Cure
EVs: 252 HP / 252 Def / 4 SAtk
Bold Nature (+Def, -Atk)
- Seismic Toss
- Ice Beam
- Thunder Wave
- Softboiled

The same Blissey as the main set, except this one sacrifices it's support move in favour of being a much more well-rounded Special Wall. It also comes with a degree of surprise factor since many players will expect Thunder Wave or Ice Beam on a single set, not both. Once you've revealed one your opponent can be taken by surprise when the other is used.

Other Optional Moves

Counter, Flamethrower, Fire Blast, Psychic, Calm Mind, Hidden Power [Water], Sing, Toxic, Wish. Counter Blissey used to be very popular, usually taking out a physical sweeper by surprise as well as making your opponent much more weary of taking on Blissey with force. Since the use of Counter Blissey has died down it can still serve as a very powerful surprise in place of Thunder Wave or Ice Beam. Flamethrower, Fire Blast and Psychic are other special moves that can be potentially used over Ice Beam on the main set. Psychic is super-effective against Gengar and Heracross, both common Blissey counters. Flamethrower/Fire Blast are super-effective against Metagross and Heracross, again, common counters. Calm Mind is a good move which Blissey can use, a Blissey with some special moves could go on the offence with it although it would still suffer from a large majority of it's present counters. HP Water had a very short stint as being a move possible to use on Blissey back when Tyrani-Boah was common place, since with Max Special Attack it could break it's Subs, thereby making it useless against Blissey. It's too situational to use presently but it's worth a mention as a nod to the past. Sing and Toxic are other statuses, Thunder Wave is more versatile but once in awhile you may want to try them on Blissey. Wish can be used over Softboiled to give your team some support.

Strategy Against Blissey

The obvious candidate for taking down Blissey is physical force, but it's obviously a bit more complex than that. Firstly, you need to account for the potential of Thunder Wave or Ice Beam, meaning Salamence can't safely switch into Blissey and most fast physical sweepers won't enjoy easting a Thunder Wave. Against TWave Ground types, immune Pokémon and Pokémon with Guts are favoured, against Ice Beam any physical sweeper who won't take Super Effective damage is favoured. Also, special sweepers that can out-stall Blissey can beat it: Celebi, Rest-Talk Zapdos, Milotic, for example (it should be noted that if you're forced to Rest Blissey can switch to an appropriate Counter to handle your Pokémon while it sleeps, so they usually wouldn't be suited Counters).

Any Pokémon with Calm Mind and self-recover can beat Blissey over time but may have problems if they're hit with Thunder Wave. Most Pokémon with 101 Subs and Focus Punch can beat Blissey one-on-one with ease, although in some cases when it has Ice Beam this isn't the case (Rhydon for example). In most cases Explosion can beat Blissey, although in some cases this isn't true (such as Electrode's Explosion which does about 60-80% depending on the EVs). Some specific moves that greatly hinder Blissey are Toxic and Leech Seed, both of which will force Blissey to switch out of play eventually, Leech Seed in particular does well with a Special Sweeper to give it some defence against Blissey, out-recovering her Seismic Toss with Seeded HP and Leftovers while slowly draining her.

Contest Moveset

A good Contest Moveset for Blissey would have to be this for the Beauty Contest best with Modest, Mild, Rash or Quiet Nature:

Attacks:

Hail
Ice Beam
Softboiled
Heal Bell

Items Attached:

Blue Scarf

Strategy Using Blissey

If you do the attacks in this following order you should have very few problems; 1st - Hail, 2nd - Ice beam, 3rd - Hail, 4th - Softboiled, 5th - Heal bell

Locations in Games

Ruby/Sapphire/Emerald Trade from FRLGXD Colosseum & XD Trade from FRLGXD (Col) Evolve Chansey (XD) Fire Red/Leaf Green Evolve Chansey

Animé Appearences

Blissey has had a couple of Animé Appearences. All in Pokémon Centers, it is in the service of Nurse Joy and it helps in the Daily Pokémon Center routine. Episode 131: Ignorance Is Blissey! Episode 173: Power Play!