Took me a while to figure out the core mechanic, and once I did, couldn't figure out how to progress. Ended up locking myself on the first floor in the end.
Sorry if I missed something obvious. I really do love the mechanic idea. :)
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Yeah, the game doesn't really teach the mechanic very well before throwing you into a puzzle.
Setting the enemy's physical defense to 0 would have worked, but unfortunately there's nothing in the area that you can copy that from. Instead, you need to copy the damage type from one of the fireballs, which would cause your sword to deal magic damage and target the enemy's magic defense stat (which is 0).
(Also, you weren't actually stuck on the first floor at the end. If you had copied another property, the ladder's destination would have gone back to normal and you would have been able to use it again)