I dropped 2 of them after 50 hours of gameplay with low level characters. Some skills appear to drop less frequently than others, or maybe it's just pure luck.

Maybe try your luck at Demetra's shop?

I think I was around the same level when I got the achievement for 40 skills. Some eneracts are really rare.

Keep looking at the shop, i got my 40 spells in few hours like this. it costs a lot tho xD

You should be able to force Demetra to reset her inventory by returning to main menu and entering the game again. It's obviously not ideal, but sometimes a man has to get creative !