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It's easy

Just find Regina in level 14, there's a hidden room in the cell (right to you where that different kind of zombie resides) talk to her

Now you need the golden key from level 17 to open it

After that go back to level 5 where you will have to fight mena, it's on the top right just a hidden wall you can phase through where "free gold" is written in a sign, open the golden cell with the golden key and you'll find her. Talk to Regina in lv 14 first or she won't be there.