Pwnemon's offensive Cresselia set is gold. Do not underestimate 75 SpA; that shit will fuck you up if you're not prepared. If you typically rely on frailer Taunters (*cough*Thundurus-I*cough*) you're in for a real treat; instead of Trick Rooming, it will instantly deal a hefty 50+% to your Pokemon, pretty much either taking you down or forcing you out. What a beast.
Cresselia is a really important Pokemon for the Doubles metagame; it makes up a large part of everything we call "Trick Room", and with Thunder Wave and Icy Wind as well, is a master of Speed control. Its supreme bulk makes it one of the best supporters in the game, as it isn't going down quickly and will make long lasting impressions on the outcome of matches. This has been mentioned but Cresselia
is really common because of this; almost everything enjoys the support that it can bring.
In terms of counter Cresselia, all you can really do is just repeatedly hit it or hit it with powerful STAB super effective moves, and both become increasingly more difficult when you face more experienced players. The great thing about Cresselia is that, like many other support Pokemon, taking it down almost always requires a loss on your side; it may not be a physical damage or Pokemon loss, but with all of the crippling tools in its arsenal, Cresselia will make sure that its killer is crippled beyond use before it itself goes down, making it frustrating to face. A lot of people make the silly mistake of assuming that all it takes is Taunt to stop Cress, and that is complete bogus.
For partners, Pwnemon already mentioned Amoonguss, which seems to be Cresselia's new bff, but
Ground-types are great partners for Cresselia too, because they efficiently remove Bisharps that try to counter Cress with Knock Off. It's a great combination because Cresselia still allows them offensive freedom; they can easily spam their Earthquakes, and even benefits from Cress's Helping Hand to deal devastating damage.
Arcticblast note: don't use Lando-T for this purpose or you still get fucked by Bisharp
Also, as a fun side note, Skill Swap Cress + Heatran is a fun combination, especially under Trick Room, where you don't have to be afraid of Ground-types that might attack you first.
