The struggle is never with people or outward events but with the states of mind that rule you from within. Principalities and powers are moods and convictions set up on high, the fixed assumptions that govern your world from the level of feeling. Flesh and blood are only where those states show themselves; the cause sits above them, in what you have accepted about yourself.
The darkness of this world is the belief that circumstance comes from outside and cannot be changed. To wrestle here is to meet a hostile state and refuse to live from it, replacing it with the assumption you want. The high places are the ruling ideas that shape everything below. A person stops fighting the outer condition and takes hold of the inner state behind it, and when the state is changed, the flesh and blood that reflected it change too.