I've just beat Ninja Gaiden Black on Very Hard, I know it's nothing to brag about, most of the members here probably beat that level in their sleep, but for me it was tough.

Anyways, here are some neat little ideas for essence for those players having trouble on hard and very hard and maybe master ninja(which I've only started).
There are some places in the game where you can get an endless supply of essence. I will only name a couple that are fool proof, safe, and still give you good practice.

One is outside Han's Bar once you've gotten into it, you have a save point, and the regenerating X-box nearby, you're safe. Everytime you exit and re-enter the bar new enemies keep spawning, you can hit the alpha one(usually the one with the shield-which gives blue essence) with a ninpo attack, obsorb the blue essence and lauch an ultimate attack yielding a higher profit for supplies, after power up at the xbox, it regenerates your health and ninpo.

There's a really good one by the drawbridge when those large trolls with the clubs are there, this one's a little cheap, but then again, the enemies on very hard mode are too. What you do is go to the drawbridge, face the trolls, now there's a line they can't cross, it's right where there bridge meets the stone ground, these trolls are restricted to that area. Now, when the trolls are far away, put one foot over this line, it draws the trolls close. Hold the Y button and prep an ultimate attack, Ryu needs no essence to do this on NGB, but it takes a few seconds, by the time a troll gets close to you to attack you, your ready to release the Y, each troll takes about two of these attack to kill them, but they drop 4000 essence everytime, and it's a little safer than going after ghost fish.

Once again, a save point is near this area, and the powering xbox isn't too far away either so anytime you need to fuel up it's there.

I hope these cheap tactics help a little, and I know a bunch of you probably figured this out for yourselves, but it's here.