Did you change anything with masteries/blessings on those champs? Faction Guardians, anything that would change stats?

It's tough to say for sure without seeing your team. Are you decreasing turn meter and getting Max HP destroyed that way, are you not timing sleeps and getting max HP destroyed too often? Are you not healing enough during the boss being asleep to counteract the Max HP that was destroyed?

Thank you for replying. I know it is difficult to tell. 🙂 I may have to try it again and pay more attention so I can answer your questions. I usually run battles on Auto, since I'm still trying to understand all the different aspects of the game. But mostly what I saw on the Info screen was HP-Burn and Block Debuff.

The team is UDK (60), Kael (60), Dreng (60), Saito (50), Ronda (50). That is the team I "came back to" so I asume it is the same team as a few days ago, when I last ran this stage. (Ronda I swapped with Fenshi ((60) a few times with the same result. Otherwise I didn't change gear or anything. Maybe I should start taking notes. ^-^ 

Maybe I just have to come back to it later and try different champs. For me was just a bit strange that I already ran this stage in 1:51 and now I have to end the battle because after ten minutes everyone (including the boss) is still in good health and fighting on and on and on....

Again thank you.

I think he is not talking about the sand devil, but the soul (green) potion keep.

The easiest solution to the problem is probably just levelling up Saito and Ronda. There are champs that can work at lvl 50, but Damage dealers don't. Any random damage dealer with magic affinity lvl 60 would be better than Saito and Ronda at lvl 50.

But beside that, an Armiger would solve your problems. The boss is not immune to turnmeter reduction. If he doesn't get a turn, he doesn't heal.

Yes, I wanted to farm some of the green potions and a few days ago I could do stage 13 without to much trouble. That's why I was confused why it suddenly didn't work anymore. I will definitly keep leveling and also keep your suggestion in mind. Thank you!🙂