A random aside -- if you turn the game on when it’s your birthday (or when your DS says it is), the game will wish you a happy birthday!
Every time you attack enemies with the Ultimate Technique, you’ll receive more Essence. Therefore, attacking with that attack will net you more Essence over longer periods of time. Try going to places like caves, where a plethora of easy enemies (such as bats) lay in wait. Using leveled-up Ultimate Techniques will allow you to farm for Essence rather easily.
There's a part of the game where you can jump into a well, leading you back to a village. There are bats in this well. Attack and kill them, grabbing what goods they drop, and then jump back out of the well. Jump back in (as the enemies have respawned below), and repeat this process for infinite funds.
To unlock all of the in-game artwork, simply beat the game once through.
To unlock a harder difficulty setting than is initially available to you, beat the game once through.