Psyduck is a FireRed version exclusive. It is fairly common; it can be found, either by Surfing or with the Super Rod, on Routes 4, 6, 10, 11, 12, 13, 19, 20, 21, 22, 23, 24, 25, as well as Pallet Town, Viridian City, Cerulean City, Vermilion City, Fuchsia City, Cinnabar Island, Seafoam Islands, the Safari Zone, and some postgame locations. Notably, it is a guaranteed encounter in Viridian City with Surf.
Although unimpressive at first glance, Golduck is far from the average Water Pokémon. Its main gimmick, which is rare in Kanto games, is the pure typing when fully evolved; this gives it only two weaknesses, neither of which is very common in the game at all, and the decent bulk contributes to making it fairly sturdy as well as a good hitter, with better-than-average coverage options. Brick Break is a decent, viable addition to the usual Surf and Ice Beam most Water-types can learn, though what sets Golduck apart the most is the capability of learning Calm Mind, as well as having the stats to support a setup. While less resilient than Vaporeon, and with overall stats slightly lower than Blastoise's, Golduck still retains its selling points and can even be one of the best Water-types to use in Kanto.
Gym #3 - Lt. Surge (Vermilion City, Electric-type): Putting off this fight for so long indicates a severe lack of options, and Psyduck is not one of them. Sure, all Lt. Surge's Pokémon have is Shock Wave for STAB, but only Pikachu can be OHKOed with Surf and even Voltorb's damage output is better than Psyduck's, not to speak of Raichu. Unless Golduck is already evolved, which leads to an easy curbstomp of the gym due to being several levels higher than Lt. Surge's ace, do not use Psyduck here.
Gym #4 - Erika (Celadon City, Grass-type): Psyduck can only OHKO Tangela with Ice Beam if a few levels higher than it, and should not come anywhere close to Victreebel or Vileplume, both of which can murder it with Giga Drain. Golduck, instead, can OHKO Tangela and 2HKO Victreebel and Vileplume; the last two only achieve a 2-3HKO with Giga Drain and are slower than Golduck, so Golduck can theoretically solo the gym if kept healed.
Gym #5 - Koga (Fuchsia City, Poison-type): The two Koffing are OHKOed by Surf. Fortunately, they are also the only Pokémon with bomb moves on Koga's team, although a Damp Golduck will find this irrelevant. Surf averagely 3HKOs Muk, though its Minimize and Toxic usage may lengthen the fight, as Golduck will need to be healed accordingly; Weezing works similarly with SmokeScreen, though Surf can 2HKO it and accuracy reductions are easily eliminated by switching out and then switching back in.
Fighting Dojo (Saffron City, Fighting-type): Surf is a 3HKO against both Hitmonlee and Hitmonchan. Hitmonlee is the most dangerous, as it can 2-3HKO with Hi Jump Kick; a prediction game can save Golduck, since it typically starts with non-damaging moves such as Mind Reader or Focus Energy, thus a timely use of Protect can force Hitmonlee to deal severe damage to itself before being finished off, but there is no guarantee the strategy will work. Additionally, since Hitmonlee's base Speed is slightly higher than Golduck's, a Golduck with a Speed-lowering nature may be outsped by Hitmonlee. Hitmonchan is slower and only 3HKOs, so there is no danger.
Rival (Silph Co.): Pidgeot is 2HKOed by Ice Beam, whereas Exeggcute is OHKOed; Growlithe falls to one Surf. Gyarados' damage output is low with every move but Dragon Rage, though Ice Beam also only 4-5HKOs; Golduck can wear it down patiently, but needs to be healed during the battle. Charizard is 2HKOed by Surf. Blastoise is boring to fight against, because Brick Break has the best damage output if Golduck's Attack has not been lowered by Intimidate, and it still takes several turns to KO; Blastoise itself, however, is not a threat to Golduck. Razor Leaf from Venusaur is a 2HKO and can even kill from full health with a critical hit, so Golduck should stay away from it. Alakazam should also be avoided; while Surf is naturally a 2HKO, its Speed is high and Calm Mind will quickly increase the number of turns needed to take it down, so it should be tackled by a Pokémon with a strong physically offensive presence instead.
Giovanni (Silph Co., Ground-type): Golduck 2HKOs Nidorino and Nidoqueen with Surf, OHKOs Rhyhorn, and can also 3HKO Kangaskhan with either Surf or Brick Break, since Kangaskhan's damage output is lower and Golduck will likely outspeed, due to its level advantage.
Gym #6 - Sabrina (Saffron City, Psychic-type): Golduck should preferably have a good physical move to hit Sabrina's Pokémon with, alongside Surf; Secret Power and Strength are both viable options. Kadabra is 2HKOed by a combination of Surf and either Secret Power or Strength, even if it uses Calm Mind once; Mr. Mime is averagely 2HKOed by the same combination, though Secret Power is less likely to KO than Strength if Mr. Mime does use Calm Mind (Surf is a clean 2HKO without Calm Mind). Venomoth is simply 2HKOed by Surf and does not know Calm Mind at all. Golduck should not fight Alakazam under any circumstance, since it can outspeed and 2HKO with Psychic.
Gym #7 - Blaine (Cinnabar Island, Fire-type): Surf OHKOs Growlithe, Ponyta and Rapidash, and 2HKOs Arcanine. Simple as that.
Gym #8 - Giovanni (Viridian City, Ground-type): Again, Surf scores a slew of OHKOs, and even in the event one of Giovanni's Pokémon outsped Golduck, a 3HKO with Earthquake is the best they can achieve. This fight is easy for Golduck.
Rival (Route 22, pre-Elite Four): Golduck benefits the most from playing this fight strategically. Since Pidgeot is non-threatening, Golduck should set up Calm Minds against it and then sweep the rival's team underfoot. The amount of setups needed varies with the rival's team; Surf OHKOs Rhyhorn and Growlithe whereas Ice Beam OHKOs Exeggcute even without any setups, though Pidgeot needs at least one to be OHKOed by Ice Beam, whereas Gyarados requires all six. Alakazam theoretically requires just two, but Golduck should have at least three under its belt, as Alakazam itself can use Calm Mind and a Psychic from it hits rather hard without any defensive boosts; with three Calm Minds, Surf will OHKO even if Alakazam outspeeds and uses Calm Mind first. One or more suffice for Charizard, whereas Blastoise is 2HKOed at best due to resisting Water and Ice moves, but cannot damage Golduck much in turn. Venusaur should be tackled only at three or more Calm Minds and with the certainty of outspeeding it, because it can survive Ice Beam anywhere below that threshold, and a critical Razor Leaf kills Golduck.
Elite Four Lorelei (Indigo Plateau, Ice-type): Since both of Golduck's special options are resisted by Lorelei's Pokémon, it will win this fight more easily with a combination of Surf and Brick Break. Surf can 2HKO Cloyster and Jynx, whereas Brick Break 2HKOs Dewgong and 3HKOs Lapras. Slowbro is the most daunting to take down, due to its resistances and bulk, but Surf can net a 2HKO starting at +2 Calm Mind, and an OHKO at +4 or more; it is advised that the Calm Minds be used before Slowbro enters the battlefield, or it will start spamming Amnesia and Yawn to delay Golduck's victory.
Elite Four Bruno (Indigo Plateau, Fighting-type): The two Onix are a joke, but the first cannot be used for setups, since it knows Roar. Hitmonlee and Hitmonchan can 3HKO Golduck at best, so they can be used for Calm Minds instead; at +2, both are OHKOed by Surf, though reaching +3 and healing before taking them down is recommended: Machamp is only 3HKOed by Surf naturally, and can be OHKOed at +3 or higher; the OHKO removes the risk of it using Cross Chop and scoring a critical hit, which may be fatal for Golduck.
Elite Four Agatha (Indigo Plateau, Poison-type): Gengar is likely to outspeed Golduck and start pulling Double Teams and Confuse Ray on it, but Golduck can break through them with healing items and (if needed) PP restorers. If Golduck manages to use Calm Mind twice against the lead Gengar, it can then OHKO all of Agatha's team with Surf or (for Golbat) Ice Beam.
Elite Four Lance (Indigo Plateau, Dragon-type): Golduck can only win against Gyarados by being healed constantly, as its Hyper Beam is a 2-3HKO and Ice Beam can only OHKO at +6 Calm Minds, as it normally 4HKOs. The two Dragonair and the Dragonite are easier, all OHKOed by Ice Beam after just one boost; Aerodactyl's Hyper Beam is a 3HKO close to a 2HKO, but a healthy Golduck can Surf it into oblivion easily.
Champion Rival (Indigo Plateau): This time, Pidgeot cannot be used for Calm Mind setups, since it knows Whirlwind; instead, Golduck should simply 2HKO it with Ice Beam, and it can also OHKO Rhydon with Surf. The rest of the Pokémon are variably threatening, depending on the state of Golduck's setups as well. Arcanine and Charizard are useful, but they are unlikely to be sent out while Golduck is on the battlefield; either way, they fall to one Surf each after one Calm Mind, and are 2HKOed otherwise. Gyarados and Blastoise each 3-4HKO at best, so they are doable for setups too, but will hit harder than their Fire relatives and make Golduck need to be healed more often before they can be swept; additionally, even at +6, they are only 2HKOed by Ice Beam and Surf respectively. At least two prior Calm Minds are needed to OHKO Exeggutor with Ice Beam, and three for Venusaur; Exeggutor's Giga Drain will otherwise dent Golduck's HP too much, whereas Venusaur can put up Sunny Day and then OHKO Golduck with SolarBeam. Lastly, a minimum of three setups will suffice for Alakazam, even if it uses Calm Mind up to twice; at +4 or above, Surf is an OHKO even after Alakazam uses Calm Mind once.
Post-Game: Mewtwo is not to be trifled with, even by a good Water-type with decent bulk such as Golduck.
Psyduck will most likely start with Tail Whip, Disable, Confusion and either Water Sport or Screech when caught, depending on its level. At level 31, it gets Psych Up. All of these moves except Confusion are varying degrees of useless; however, as Surf will be already available by the time Psyduck is obtained, there is an easy fix for the lack of coverage. After evolving, at 33, it gets Fury Swipes at level 44 and Hydro Pump at level 58; only the latter is actually useful, and only if Golduck must improve its Surf damage output, but Surf is still a very valid choice for the entire game and Golduck will not miss out on much by not replacing it with Hydro Pump.
TM moves are what makes this line appealing, as its coverage is better than the average Water-type's. Calm Mind is an option for setup sweepers, as Golduck's bulk and typing allows it to take hits well enough to get a couple boosts off, at which point OHKOs are not terribly hard to come by. Ice Beam is a must, or Blizzard failing that, but Ice Beam is definitely better and will work as an all-purpose Dragon killer in the late game, as well as providing general coverage. For more stall-oriented science ducks, Toxic is also an option, and one that Golduck can use efficiently at that. Lastly, for physically-oriented Golduck, the all-popular Brick Break will definitely work well, and Dig can offer support against Poison-types too, although Surf will still probably deal more damage (unless Golduck has low Special Attack and great Attack). Surf is an absolute must if Golduck opts out of Hydro Pump, and/or before Hydro Pump is learned.
Grass-types: Grass covers best for Golduck's weaknesses, especially considering that all good Grass-types available in Kanto have a secondary STAB to hit other Grass-types with. Their Electric resistance is also very valuable for Golduck.
Specific Pokémon can also provide Golduck some help against Grass- and Electric-types, if a proper Grass-type team member is missing. Nidoqueen and Nidoking are neutral to Grass and immune to Electric moves, for instance, while Dragonite provides better coverage against Grass and can learn Earthquake to counter Electric-types.
Minimum stats are calculated with 0 EVs, IVs of 0, and a hindering nature, if applicable.
Maximum stats are calculated with 252 EVs, IVs of 31, and a helpful nature, if applicable.
Psyduck Line Ratings
What Nature do I want? Anything that lowers Attack and/or boosts Special Attack. Modest is ideal, but natures such as Bold and Calm also work very well. Timid can improve its Speed, at the cost of sacrificing Brick Break power. In general, if Golduck's nature lowers Attack, it will benefit the most from running Calm Mind, whereas neutral natures and/or natures with other penchants will favour mixed sets (though they can also run Calm Mind, only slightly less efficiently).
Which Ability do I want? Given that the Pokémon with weather moves are not very common in this game, and/or that Golduck itself may want to use Rain Dance, Damp is the best ability to have. Protection from bomb moves is always a plus, and helps defeat Koga more easily.
At what point in the game should I be evolved? Anytime after fighting Erika is best, since neither Psyduck nor Golduck will be very useful in there, exception made for a Golduck carrying Ice Beam. Evolving Psyduck drastically improves its survival chances, and there is no boss fight that Psyduck can do well in after Erika.
How good is the Psyduck line in a Nuzlocke? Very good, and also very underrated due to the omnipresence of Water-types. In Kanto, Golduck has a great niche - it works incredibly well in teams of Bulbasaur and Charmander pickers, or people who chose to not use Vaporeon. Unlike Vaporeon, it is also fast enough to hit first in a number of cases, and can run physical moves alongside special ones.
Weaknesses: Grass, Electric
Resistances: Steel, Fire, Water, Ice
Neutralities: Normal, Fighting, Flying, Poison, Ground, Rock, Bug, Ghost, Psychic, Dragon, Dark