The quickest way I can offer is first reach unholy level 7 by punishing the enemies you come across, aim for the shades, gluttons, and demons as they will give you more unholy expierence. When you reach unholy level 7 try to absolve the enemies you encounter as this will net you more souls then just punishing or killing them most importantly don't auto absolve shades, but do the little mini-game to get the extra souls and just keep adding them to the unholy path. I almost completed this in one playthrough, so it isn't that hard just a little time consuming. Also ABUSE (if you want to) the infinite respawn battles such as when your fighting cleopatra on the elevated platform that she is holding on to where there are always three babies that she throws on the platform.
If you are willing to use MSP then you can buy some of the soul packs and there is a free soul pack even though it has a measly 500 souls but it is free so who can argue.
EDIT: Using Charon's Oar(Holy) and Tail of Minos(Unholy) relics will give you more souls for punishing and absolving enemies. Gates of Hell helps you net alot of easy and quick souls on Casual difficulty as well.
Thanks to DownfallEclipse for this extra information.