I don't get why nobody mentions the blank card. Easily my favorite in the game, especially when you manage to find a sun or joker early on. I found a chaos card the other day when I had it Every boss died in a single hit. That's how I killed the Dark Room boss with ???
That's just the beginning. Now, you'll be able to get to level 7 (rather than 5) and the boss there is WAY harder. After that, there's even more bosses that make the first 5 floors you beat increasingly look like a tutorial stage