Made it to lvl40 today. I got no clue how to beat the boss there. But I thought that of other 'roadblocks' on the way too. Don't forget this is an idle game. So be patient and use the dark ritual when hitting the ceiling!
I'll got to cave 21 just now after my ~5th dark ritual. The first 15 lvls are a piece of cake now, dark mana constantly growing. At cave 21 there is special loot: 'unique abilities for your heroes. I'm looking forward to beat it's guardian! ;)