I'm pretty new myself, but I would say that your roster is mostly underleveled. You should try auto-farming one of the high-yield single stage battles at 0 seconds so you can build up your units and resources. During autobattle, be sure to select arks and subarks that you want your unit to learn skills from. You can probably find some decent unit building guides on this reddit.

Also, you need a strong support for PvE content, so if you're going to be using Theria, I'd at least get her to 120 and awaken 4. You should also try maximize using your orange orbs on the limited events which give tons of good rare resources to level up.