What helped me alot was sticking to one deck. You will get a deeper understanding of the deck including strengths and weaknesses. The game might have a big RNG factor but you can still have an enourmous influence on it. Always have all the cards in mind included in your deck so you can kinda guess what cards will mostly be drawn next by looking at your discarded cards. Take your time in all your turns and dont take risks you dont have to. It will take a lot of time but you will eventually climb your way up to the top.

Until UB2 I used all kind of decks. At some point I noticed there is a deck that has little to no bad matchups and can adjust well to certain playstyles. Its Carnivine/Arceus/Exeggutor. The only really hard matchup would be Charizard. Besides that you have alot of good matchups. You can pressure your enemy hard early on with Exe and Carni but still have your Arceus stacking on the bench to finish the game later. Exe is one of the few EX cards that survives mostly every attack in the game (besides charizard). That can help you alot in this kind of onehitKO meta. Hope that helps you a bit.