Excellent game, but room for improvement
I thought this was a great game. I just finished playing through it on easy mode. I really liked the creative selection of spells and I give you kudos for creating an innovative way of adding those spells to the game (or at least I thought it was innovative, because I had never seen a spell system like that before).
The story, while not great, was enough to carry me through. The graphics were generally really nice, though the ice shards and meteor spells had somewhat disappointing graphics that didn't seem to match the rest of the game.
Also, nice move including a boss at the end. I think having a good boss at the end makes a game much more satisfactory. (And nice job making him immune to polymorph. I thought I could cheat my way to victory by taking polymorph into the last level, but you were too smart for me. :P )
I don't know about the other difficulty levels, but the difficulty curve on the easy mode seemed to be set pretty well - that is to say, I was able to beat it on my first try, but I was challenged along the way, and nearly lost a few levels, which is what I would hope from an "easy" mode (not too easy, but just easy enough), so nice job there, too. You seem to have a good grasp of the difficulty of your game.
And I liked the luck/strategy element involved with the powerups. They were usually a blessing, but if I accidentally hit one while trying to cast stonewall or firewall, then my spell would get used up, and it would be replaced with the powerup, so the spell I actually chose would never appear. That was a clever element to add to the game.
On to some areas for improvement...
There was a graphic glitch. After a level ended, I would typically see things like my stone wall, my arrows, my health bars, or those falling power ups on the "Upgrade and Memorize" screen that occurs between levels. While this doesn't impede gameplay, it's somewhat annoying. (I have the latest version of flash player, by the way - I just updated today)
There was also a story glitch. After completing the "Doom Lands" level, the story it displayed was the same one I had received after completing Dark Forest ("Many slaves are released and are reuinited with lost loved ones..."). Although your game didn't really hinge on the story, it would have been nice to see the last part of the story prior to entering the last battle.
Finally, I'm going to voice a complaint that was mentioned several times in previous reviews, but I feel deserves repeating. It takes excrutiatingly long for some levels to end after all the enemies were gone. At first, I thought it was a bug, but after waiting several minutes I realized it was just a long level end timer. I'm not sure what causes that timer to start running, but you may want to look into shortening it providing it doesn't adversely affect gameplay. On the plus side, your background graphics were REALLY nice, and they gave me something pretty to look at while I was sitting there doing nothing waiting for the level to end. :)
Oh, and one more minor issue. Before you unlock the polymorph spell, it says that it changes enemies into sheep. But after you unlock it, it says it changes them into pigs (which is what it actually does). It's no big deal, but I like sheep better than pigs, so I was mildly disappointed when I found out that it was pigs and not sheep (though it was a really cool spell either way).
Overall, great game. This is definitely front page quality material. Keep up the good work.