In the end, what matters most is the strategy you will deploy with your Pokemon. Even if you don't do IV and EV training, it's best to create a plan on how to deal with a battle. For that, you must understand the Pokemon, like the moves it can learn, strength, and weakness. A Blissey has a high HP and DF value and can learn Soft-Boiled, making it like a "wall" and causing your opponent to waste PP. Some Normal-type Pokemon, like Stoutland and Lopunny can use a diverse type move that would typically need a certain type Pokemon to have, like Thunder Fang and Fire Punch.
Cleverness also counts, because strength alone will get you far. Case in point, a player who won the battle by using a level 1 Pokemon. How is it possible? It was a Rattata who was born with both the Endevour and Quick Attack move, and was holding a Focus Sash. All it takes is a hit, use Endevour to lower the target's HP to 1 and use Quick Attack to move first. Of course, that would never happen again, but using these clever tactics will be a huge benefit. That's true with Double and Triple battles when using the right move combo. You can look it up yourself.
Remember to study more about each Pokemon, have a variety of type moves, and be clever with your plan. I hope it helps.