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>Moves
The more Egg moves you can jam in, the better. Even though 99% of the people aren't going to use them, Acid Armor, Counter, Curse are probably the 'best' and are still shit
>Ability
Breed Gooey. You'll get plenty of non-Gooey for people who want non-Gooey
>IVs
Max HP, Defense, Sp.Defense, Sp.Attack at the very LEAST. Min Attack is useful if they aren't going to be mixed, and min speed is okay for Trick Rooms, but I'd say max both of them anyways.
>Nature
SpAtk+. Modest for non-TR, non-mixed, Quiet for TR-users, and I don't know what for non-TR mixed.