I know this was kind of a biased strategy, but I thought it would be worth sharing.
I personally don't know much about EV's and IV's except what I have learned by playing Pokemon Go, that includes nature as well.
I have completed generation 1 to 5 and I have gotten all the current badges in Reborn, so I don't consider myself bad at Pokemon, but whenever I try to battle others in Reborn it usually doesn't end well for me.
I have tried getting stronger Pokemons for my party with high average stats and great offensive moves, but it doesn't seem to work.