High IVs & EVs and a good Nature
>Rather than just being a superficial personality, Natures actually affect the growth of a Pokémon. Each nature increases one of its stats by 10% and decreases one by 10% (by the time it reaches level 100). Five natures increase and decrease the same stat and therefore have no effect.
>Individual Values (IVs for short) are key to determining your Pokémon's stats. IVs are randomly assigned when you encounter any wild Pokémon (or receive it as a gift or egg). You cannot change a Pokémon's IVs once caught and there is no way to know the exact IVs in-game.
So you cant change these. (Sorry) If you've got a Gardevoir with 31 Special Attack IVs you can still max it's attacks.
>As Pokémon battle, in addition to gaining experience points they also obtain Effort Values (usually referred to as EVs), which are special hidden values in each stat (HP, Attack and so on). Each Pokémon you battle against gives off EVs in one or more stats.
>Generally, this relates to the Pokémon's strongest stat - for example, if you battle a Geodude your Pokémon will get one EV in the Defense stat. If you fight a Gengar, you will get three Special Attack EVs. (See the pages to the left to see which Pokémon give off which EVs.)
So you can change these by doing a thing called EV training. You can learn about it here.
If you have maxed special attack on Gardevoir you should have 383 special attack.
Good luck EV training