Level difference start mattering less and less, so the difference between the player at the start, and at the end of the combat are much lower. You should expect balancing issues to exist in new dungeons more often now.
One way for balancing in later levels would be the use of the unique dungeons mechanics. E.g. you can only hurt the Boss or prevent some bad mechanic, if you reach a certain point of transformation, a specific status or got a high amount of a specific stat. Generally speaking some condition the player has to archieve in the dungeon to be able to beat the boss. Something similar you did before e.g. in dungeon 1 with the shaming / shamelessness mech or the corset/heel trainig in dungeon 2 . In the pony-dungeon at the moment you can enter the dungeon, run to the boss straight on, beat it and leave.
Another possibility for balancing I could imagine would be to use the fixed level-window per dungeon. E.g. in Dungeon 7 you are something between lvl 28 and lvl 30. You could add a special skill the girl (or just one) gets on lvl 30 you need to beat the endboss of that dungeon. This type of balancing should only be used selectively.
One other point: Is there a possibility to add more save slots? I used to keep a save game at the state before entering a new dungeon. And use 2-3 slots within the dungeon.