Oh, the game saved automatically when the murga fight started and she did 400 damage (my armor) then 550 damage (my HP) in one turn.
I know she does this much damage, but I thought I turned off autosave. And the sad thing is, everything else was so incredibly easy and intuitive I would have beaten it but swore I would never try again if I somehow died to something this time. Even dying during the final fight would be preferable than dying because the game destroyed your save file.
