In standard, Golduck looks good on paper. Firstly, it isn't affected by Tyranitar's Sand Stream, which often ruins Pokemon that try to pull off a Substitute set. Golduck can also can get rid of special walls thanks to its Fighting-type moves and still attack decently. However, Golduck is KOed quickly by standard sweepers and stopped by a lot of walls, which makes it hard to use.
Fire away with special attacks, and when your opponent brings in Blissey, Snorlax, or Regice, use Focus Punch or Cross Chop. While Cross Chop is only a 3HKO on Blissey, its high critical hit rate gives Golduck a good shot at a 2HKO, or even OHKO with Spikes down. Psychic 2HKOes Heracross and possibly OHKOes Gengar, while Hidden Power Electric hits enemy Water-types, especially Gyarados.
Some Attack investment is needed to hurt the standard special walls, but Golduck doesn't usually need more than 220 Attack. The Speed EVs let Golduck outspeed unboosted Tyranitar, allowing it to take Tyranitar down with Cross Chop or Surf; the rest are put into Special Attack to enable it to sweep.
Hydro Pump is an option over Surf for more power. Toxic cripples other Water-types and forces Regice and Thick Fat Snorlax to Rest. Refresh can let Golduck heal itself of status, but one is better off preventing it altogether. Psych Up enables Golduck to snatch Calm Mind boosts, but Golduck isn't likely to win a Calm Mind war because its stats are so mediocre. Light Screen is in itself a good support option, but Golduck isn't the best user of it.
Checks and Counters
Zapdos is one of the best counters to Golduck. In addition to packing a super effective STAB Thunderbolt, the common RestTalk Zapdos doesn't mind Hypnosis, while most defensive sets can take several hits—even the super effective Ice Beam. Raikou can do the same, but while it lacks a weakness to Ice Beam, it rarely has Sleep Talk.
Blissey, Snorlax, and Regice are good Golduck counters as long as they avoid Focus Punch and Cross Chop; in particular, they counter Calm Mind sets pretty easily. Additionally, Regice and Snorlax can both 2HKO Golduck, the former with Thunderbolt and the latter with any STAB Normal-type attack.
Celebi takes less than 50% from an unboosted Ice Beam and can use Leech Seed or set up Calm Mind boosts, then hit Golduck super effectively with a STAB Grass-type move. Suicune can also Calm Mind up, as well as phaze or poison Golduck without taking much damage. Milotic takes little damage from any of its moves as well and can use Hypnosis or Toxic on Golduck or set up Light Screen.