I agree! In my challenge I used Shido (Shiftry), Akira (Mighyena), Casimira (Absol) and Scylla (Sharpedo). However, to defeat the League, I caught Valruna (Mandibuzz) too.
I've actually played up to Winona's Gym with two Mightyena's and two Nuzleafs due to lack of variety in dark types.
So, what brought you back then? :) by developing a Pokémon you mean EV train? That's been a feature since long ago, it's just made easier in the 6th gen. ^^