I use mine competitively in my ORAS Battle Box, it functions as a destroyer of weather activators:
Golduck @ Zoom Lens
Ability: Cloud Nine
EVs: 252 HP / 4 Atk / 252 SpD
Sassy Nature
IVs: 0 Spe
- Aqua Tail
- Hypnosis
- Wonder Room
- Focus Blast
My Golduck is built to take out the Pokemon whose abilities activate a weather condition (Drought, Drizzle, Sand Stream). I did the research, and Water- and Fighting- moves have the best super-effective coverage (ex. Tyranitar, Hippowdon, Ninetales, Mega Charizard Y, Abomasnow, etc.). Since Cloud Nine negates the effects of weather, these two types of moves hit for full power -- seeing as Sandstorm and Sun would usually raise Rock-type's SpD and Sun would reduce Water-type damage.
I chose a physical Water-type move because a majority of the weather activators that are weak to Water- had lower Defense than SpD. I chose Focus Blast because a majority of the weather activators weak to Fighting had lower SpD than Def. For the weather activators whose other defense is higher than the majority, I chose Wonder Room to swap their defenses and proceed to take them out. Hypnosis is to put the weather activator to sleep so you can comfortably proceed to KO them.
Max HP and SpD is necessary to prevent being OHKOed by Special Attacking Electric- and Grass-types. Since Golduck's Special Attack is higher than its Attack, it makes sense to teach it a non-STAB special move for maximum power and a physical STAB move.
