When Goduck use only water-type moves, Venusaur is the best to beat it. While reducing damage of water-type moves, Venusaur's grass-type mvoes are super effective against Golduck. Additionally, Venusaur has higher CP along grass-type Pokemon. Note that Golduck can also use psychic-type fast/charge moves which are super effective against Venusaur.
Althoough Venusaur can learn Sludge Bomb, players should get Venusaur have all grassy-type moves When fighting against Golduck. The combination of Vine Whip and Solar Beam is considered as the best moveset for Venusaur as an attacker.
When Golduck's moveset is the combination of water- and psychic-type moves, Magnetom is one of the best Pokemon to fight against. While decreasing damage of psychic-type moves, Magneton's electric-type moves are super effective agianst Golduck. Note that Magneton is neutral against water-type moves.
To defeat Golduck faster, Magneton should use electric-type fast/charge moves.