As described here, on average, every 3 floors a character will have leveld up 5 times (1.666 levels per floor). This means that by the time they reach the final boss at the 15th floor they should be around level 25.
I want every character to get 10% stronger per level, meaning that by the time they reach level 25 they will be 250% stronger than at the start of the game.
Balancing the game will be hard due to the fact that there are so many variables (character stats, equipment, spell upgrades) but I believe it will be doable once we have a playable prototype where its possible to adjust the numbers before each playthrough.
While this might put some players off, the game will be designed for optimal play because with time players will get to that state of mastery. If the game was designed to be easy it would get boring after a few playthroughs.
Since there is no specific order to when the enemies are found in the dungeon, every enemy must be balanced according to the starting point of playable characters and then scaled to the floor the player encounters them.
Boss enemies should be designed to be harder and provide a big challenge.