I may just be bad at this game, what's the secret?

How is everyone even finishing Act 2 and more, let alone Act 1? I played 50+ runs already and got halfway through Act 2 like 5 times.

Maybe I'm just ass at putting a deck together but some of these bosses/elites deal 20-30 damage back to back. I'm trying to block to avoid death but I also need to attack cause the boss has a ton of HP and gets stronger with every turn. How are people getting through everything, what's the secret?

This is my first time playing Slay the Spire. Game is addicting but I'm struggling. The common battles are easy but when it comes to elites/bosses I fail hard.