TIPS AND SPOILERS AHEAD!
Things that made a click in my head and I was immediately kicking monster butt:
- Understand the level up and the heart scroll as 2 ways of healing and use them both ONLY when at 0 hearts, since the gems you get for klling monsters keep accumulating!
- Don't focus on killing monsters at first, just like in minesweeper focus in finding empty cells by looking at the numbers, attack only when no other option available, when you need to lvlup or when you are powerful enough to kill a boss and get the reward.