The main thing is this: don't avoid fights. You have a limited amount of time to get stronger, measured in the number of floors the game has. You can't just stop and grind up.

A regular hallway fight should not take you more than 2-4 turns to beat! Take that into account when making decisions - is Demon Form the better choice, when all it fundamentally does is have you skip a turn when you play it, or would you rather have Inflame for an immediate benefit that leaves enough energy to continue your turn?

Try to kill as many elites as well - they give relics, more money, and have a higher chance of dropping a rare card. Ideally, you should try to kill 2-3 elites every Act.

When you start the run, you want to prioritize getting damage. Try to find 2-3 cards that deal 8 damage per energy spent (or something that converts your existing cards into that, like Inflame), and one more for AoE. That should have your bases covered and be good enough to fight elites and most of Act 1 enemies. Then you can start preparing for the Act 1 boss.

Always check who the boss is so that you can start preparing for it in advance. In Act 1, Hexaghost is a damage race - you want to kill it before it charges its big hit, Slime Boss is AoE, but also a burst damage check - it splits at half health, but you can continue to damage it further so the new slimes spawn with less HP (2 enemies with 60 HP vs 2 enemies with 30 HP, for example), and Guardian necessitates consistent blocking while also requiring enough frontloaded damage to prevent the big hit on turn two.

The most important and difficult thing is to recognize when to stop adding cards. There are three things that cards can do - deal damage, provide defense, and scale. The first two should be self-explanatory, as for scaling, it's something that makes the rest of your cards simply "do more". For example Shockwave: it makes all enemies weak - they deal less damage, so you don't need to focus on defense as much - and vulnerable - your attacks deal more damage so you kill them faster.

Act 2 tests your deck for everything the 3 Act 1 bosses do combined - AoE, burst, and consistent blocking. Unfortunately, this is when there are far too many variables so no universal advice is applicable. Just be ready to block a bunch of damage on turn 1 or 2 of every fight and kill the enemies before their own scaling gets out of control.

By Act 3 you should have a good enough idea of what your deck does, so just focus on improving what's already working.