You lose because you run out of "Corruption" (the mana for your dungeon) by Floor 4. Here is how to conserve.
If you are losing, ask yourself: Which phase did I rush? hero party must fall guide better
Put your strongest damage dealer in a defensive gear set (temporarily) for the first 2 turns. The enemy AI targets the hero with the highest Attack stat. After they waste their cooldowns on your “bait,” swap gear back (if you have quick-swap unlocked). Better yet, use a second gear preset. You lose because you run out of "Corruption"