A few people have gotten stuck on the sun and moon path. The way it works is the party has to think through the clues while doing something else. If you've already done everything else in the game, then there's nothing left to move that dialog chain forward. There have been a few players who were in the same situation, but they managed to find something else to do to unlock the next step. I need to redesign this part. Maybe beating the diamond tournament will be enough "progress" to trigger the next scene.
Umbra's father is one of the most difficult battles. If you can beat him, there's only 1-2 battles left that can challenge you.
He has a lot of healing from life siphon and the healing skeletons he summons. This can be countered with a well timed use the Zombie Flesh accessory to reverse healing.
Ultimately, the party should be level 60-70. There's a few more bosses locked behind the story that will level the party up.
Here's a video of a player doing 100k+ damage in a single turn. Their build can give inspiration on how to deal more damage too.