Take a good look at your deck of cards (in the deckbuilder) to see if you can optimize your deck and/or buy new cards at the shop. You can go back to the previous dungeon to gather more gold. Also use the run away card to your advantage.
That's because it is. I also struggled a bit at the second dungeon, until I understood you're supposed to replay each dungeon a couple more times to grind gold and buy upgrades. Also there's a small tactic you have to understand: you don't have to beat every opponent in each turn, since you can always leave one card behind...