You are correct that for the pain skull the % goes down by 1 to 3 % after each battle. And it is totally RANDOM. Save before each fight and reload if you don't get 3%. That'll work. That way you can end up with half mana for only 34 battles (at best).
If you left any weaker opponents in previous areas, this would be a good time to clean them up and get that % down quickly.
KD
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