I want to Masuda method an Azurill for a shiny one, and I have one set it perfectly (not shiny). It has the best nature, ability, and gender (Azumarill gotta stay manly). I know how to carry down a nature (Everstone), but how do I guarantee ability and gender?
Gender is random and is based on the gender ratio for that species. For the Marill line, this is 50:50. There is no way to influence this.
As of Gen VI, a female pokemon has an 80% chance of passing down its ability when bred with a male pokemon (ie not a ditto). Male pokemon have an increased chance (60%) of passing down a hidden ability when bred with a ditto but there's no easy way to pass down regular abilities (I assume you want huge power) with a male pokemon.
You could fix this by breeding a female marill with the characteristics you want and using that, giving you a 40% chance of getting a male, huge power marill. This may also require Game of Thrones levels of incest.
Starting in Black2 and White2, there's an 80% chance the female of the breeding pair will pass on her ability to the egg. However, it appears that if the ability in question is a hidden ability, the chance is 60% for the female to pass the ability onto the egg.
If it is a male (or genderless) with the hidden ability, the only way to breed for the ability is with a ditto (I believe the chance is still 60%).
There does not appear to be any way to influence the gender of the offspring - that is determined on a species by species basis - Azumarill's gender distribution is 50/50
So what does this mean?
- If your otherwise-perfect Azumarill has a hidden ability (Sap Sipper) that you want passed to the (eventual) shiny offspring, you'll want to breed it with a ditto*. You'll have a 60% chance of the offspring having the hidden ability, and 50% chance of it being male (the everstone guaranteed 100% on the nature). Ultimately a 30% chance that the Shiny will be a male with the correct ability.
- If your otherwise-perfect Azumarill has one of the two base abilities (Thick Fat or Huge Power) you want passed down, you'll want a female Azumarill* with that ability. This will give you an 80% chance of the shiny having the ability and, again, 50% for gender. Or a 40% of having both.
*in either case, your Azumaril will have to be from a different region than the breeding partner for the Masuda method to work.
(all percentages based off of Bulbapedia and assumed to be correct or at least close enough)