I enjoyed playing your game, but that second boss is super OP! Maybe I just didn't notice a weakness, but it seemed that his shield never drops for more than a fraction of a second at a time. I think a game like this would also be improved if you lowered the number of enemies, but got rid of most healing and gave the player a finite number of hits (say four or five) so that it doesn't devolve into seeing if you can just heal through all of the enemy damage.
Were you inspired at all by Ziggurat? It reminds me quite a bit of a top-down version of that.