Not to mention that even if you have a good deck, it doesn't mean you are going to win. I just got first the last tournament i went to with my beedrill deck. Not even mentioned anywhere in tiers. Its really the player and not just a good deck. Play choices are more just as important as the right build of a deck.
An example of this is that there was a player that had a keldeo/blastoise deck, with a horrible deck construction. Like 20 water with DCE and like no skyla (which imo is really important for 2 step decks). I destroyed it with my beedrill deck. Then for fun we switched decks and i still managed to beat that player with the horrible deck they made.
So basically, just make the right play choices, and even with not so competitive decks can do well. If you need help with that and even deck constrution. I would suggest youtube, watch other good players. Especially the Topcut channel. The player featured mainly on that channel is pretty amazing and wins with terrible decks, just because he knows the correct play. What makes it better is that he goes over what hes thinking while hes streaming it. So not only do you know the good plays, but you know why they are good.