We aren't tiering Pokémon based on how they perform against others; we are tiering them based on how they perform against the trainers in the game. It is possible that one Pokemon completely outclasses another but both sweep every trainer in the game easily. In this case, both Pokemon would be high tier. This is what Super Jocke was trying to say earlier... Regardless, Golduck just doesn't have the super-effective coverage to tackle the game single-handedly.