Once you're in the game and want to upgrade your abilities with your soul, forget the power stats. This doesn't help at all, as the main source of damage is magic. Focus on improving your magic skills as high as possible.

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Since the cost of a soul is relatively low in the first few levels, it's ok to plant enemies early on. However, the higher the skill, the higher the cost. Enemies don't drop a lot of souls, so it might take some time.