Progress in Season 1 depends not only on hero power but also on your water percentage, visible top left during the adventure. Only the water generated from the Spring of Nature building counts toward this.
If your water percentage is equal to or greater than the requirement for that zone, you fight at full power. If it’s lower, you’ll suffer extreme debuffs — up to -100% HP and damage.
To prevent this, level up the Spring of Nature using Ancient Tree Branches found in chests.