I stuck with focusing on the Succubus skills, and I somehow got a Drain weapon, indicated by the heart icon, that made the extremely long post-game labyrinth so easy (Easy Mode). Though, I messed up at like 80 floors.
Unfortunately, there was a major bug when the Skill Point Up item refused to work, so I couldn't use any more than 3 Skill points in 1.00. And with the H-gallery, it's omitting a huge chunk of animation variations unless this is merely an oversight by the developer.
The difficulty is as summarized for anyone unaware:
Easy: Level and Skills permanent
Normal: Level resets, Skills permanent
Hard: Level and Skills reset.
The dev must really wish they could use something else to optimize the game, so it wouldn't be a pain in the ass to add more variations. Not that it's any easier to solo-dev on a different engine, though.