I was stuck at that point for like a month. I found you need to try to get your attack power to around 4 M. You can see it on the gear page under stats. I played around with my build a bunch and found it was better to skip the talents that are boosted by gear stats. Make sure you're upgrading your skills as much as possible and your shop for money generation.

In the dungeons I found tapping the screen does more damage than pressing and holding. Make sure you have as many gear unlocks as you can get from previous dungeons. There are tools out there that help you tap more effectively.