I like the game concept and the progression of enemy combinations. I liked how each enemy had its own distinct characteristics. Maneuvering to dodge or ready for attack feels pretty good.
The player's attack has a very slow rate of fire that feels annoying. I'd rather have a more frequently usable attack that hurts a smaller percentage of the enemy's health.
If you really want to keep the slow player attack, I suggest visibly showing the player wizard charging up a fireball in its hands (like sparks flying toward the hands), and then show a fully charged fireball when ready to fire. The way it is now is so slow, with nothing happening, it made me wonder if the attack was bugged.
The transition screens between levels are annoyingly slow. It also made me wonder if my input was bugged.
The colors remind me of Nintendo's Virtual Boy, or the Infrared Goggles in Metal Gear.
Your sprites aren't bad looking at all. I like that you restricted yourself on colors because it forced you to give each character a distinctive silhouette, which is always important even if you are doing characters with more interior detail.