Blissey is a big bulky pokemon with massive sp.def and hp and can take any hit unless it is a physical attack.
Name: Offensive Rain Set
Move 1: Thunder
Move 2: Ice Beam
Move 3: Softboiled
Move 4: Charge Beam/Calm Mind
Ability Serene Grace/Natural Cure
EVS: 252 HP / 252 SpA / 4 SpD
Excellent for dishing out damage while getting paralyzation and freeze hax if chosing the Serene Grace Ability. Can dish out damage with charge beam while always gaining a SpA raise as well. Able to take SpA hits while having access to recovery. Can take out walls such as Skarmory and Gliscor, and can take out pokemon who switch in such as Dragonite and Haxorus and can take them out as they try to Dragon Dance.
Calm Mind is just to take hits froum focus blast better, but charge beam + serene grace is the viable option, as there is a 60% paralyzation chance from thunder and 20% from Ice beam.
Teammates and Counters:
Any physically defensive pokemon that is able to take loads of physical hits (especially fighting attacks) and able to recover HP is a great pokemon to work with Blissey, such as Gliscor.
Blissey fears pokemon with a high attack stat so it cannot stay in. It can also do very litle to ferrothorn as ferrothorn can leech seed it, gyro ball and power whip, which will be doing a lot.
Blissey is great to have with a pokemon weak to ghost as it is immune to it-as mentioned blissey should be added to anyone's team that is abusing rain and doesn't have many pokemon who can take special hits.
[COUNTERS] Strong physical sweepers, or strong special attacking pokemon that have access to psyshock can OHKO Blissey-even pokemon with no stat boost can 2HKO if it is a physical attack.
Any Fighting type, focus blast will still be doing a lot as this Blissey has no investment in special defense.
An opposing Blissey or Specially Defensive Chansey that can cripple serene grace sets with Toxic.
Blissey gets the dream world ability healer, which is a pretty good ability. However, it should only be used in double/tripple battles and only on a stall set. Even in double/tripple battles natural cure is the best option because blissey should be kept as healthy as possible.