actually, the bosses get easier, although the average enemies get way harder.
Want a quick way to make a lot of money in the game? I've got a "cheat" as it were... (developed myself).
There's a point at which you're in the sewers, and there's a tunnel you fall down (just before the first electrical worm). It looks like a chimney and it's filled with ~10 bats.
Now, this exploits 3 different elements.
1) charged up combos (using the floating essence) increases the multiplier on the essence found (and ensures it's all yellow essence).
2) Bats respawn when you've gone far enough away.
3) Damage taken from bats does not interrupt your charge.
Equip normal shuriken and vigooran flail.
So what you do is fall to the bottom of the chimney, and run away from the bats till you get to the clearing.
Turn around and head back to the bottom of the chimney, and throw 2 shuriken (one at a time, don't throw more than 2). Two bats die, and you can max your essence charge. Now charge up, and wait until all the bats from the chimney swarm above you... there should be 10 at a time, but you won't see all of them at once. I suggest waiting until you've been hit 4 times. (STILL HOLDING CHARGE). Now, unleash. BOOM! x9 combo and big essence. You could pick it up, OR throw one shuriken and hold down charge again (pulling the big essence and the little one to get a full combo hit. You get 3 sets of bats, and then head out to the clearing, and repeat the whole process over. In under 45 min I was able to max out my essence twice and buy all items / upgrades, including the wooden sword.