Hello,
Story can generally be beaten with 1-3 lvl 100 pokemon. At the end of each region you can use the pokes from past regions on the E4 so each region just get 1-2 lvl 100s and you should be fine. To save time in the long run though hunting proper pokemon with natures and abilities and ev training them is good too. You can do either way in the end play it how you want to.
Have a good day :)