I'm playing 5 days so far ... and in easy mode, mostly. Never beat it. The game need a serious logic rethink.
A: vampires = game over. Leave them for harder difficulties. Really.
B: Ogre that cover units = defeat single ogres. Too costly on energy. Solution? Rush the stage. But you'll end to have more ogres AND more kind of shuffled single monsters. This can be solved by some kind of item, but is too random. My suggestion is to have ogres cover only horizontally.
C: Ice rod = why? really. I want to remove enemies, no need to stop them. Fishing rod is unusefull.
D: Level 6 onward= wrong math. More variety of monsters mean less groups, so more damage on your side to solve the stage. PLUS more deadly enemies... PLUS 3 hits to open the door (usually on single enemies that drain you 25% of the energy). This is not hard, is impossible... if you don't own the correct items (that's too random). So: or more variety, or more damage - one of them, not both.
E: Difficulty progression. My suggestion. Easy - really easier than now, but no prize or endings, and play alone. Normal - as it is, and up to 2 characters, unlock hard mode. Hard - 3 characters, unlock some other game mode (survival?).
F: Knight= another skill, like blasting in one hit multiple hit enemies (orcs, turtles...).
G: New Characters to unlock in normal and hard mode will add some replay value (Cleric, Elf...?)