Black Fire Orb was my go-to spell, since it does really good damage and the fp cost is negligible. Affinity does massive at higher levels, but the FP cost was a bit too high for my taste so I only used it for PvP.
Great Deep Soul is ok for slower enemies and to spam against invaders when they're distracted, but not much more than that. I do recommend using it against N
K
first phase though. You can take his Drake out in less than a minute with headshots.
In regards to Dark Edge, I wasn't really a fan. It seemed to do the same damage as the Black Orb with more FP cost and it's a Vertical slash so it usually misses.
Black Serpent is really good in tight corridors, since it can hit multiple enemies at once. It's almost useless out in the open though.