Not bad
First the graphics are awful, seriously you should have just ripped some sprites or something, no matter how generic it would be a big improvement. Even stickmen would be better.
But this isn't bad. The fight goes on a bit too long, and I would have liked warning that this was a boss that could change his weaknesss, but it drastically spices up what would be an otherwise boring fight. I think you should add a health meter for the boss, so we can see how closse to death he is.
Also adding more skills (ones that would effect stats might be intresting, and ones that are situatinoally good or useless) would further improve the stratergy of the game.
Don't get too discouraged by the lukewarm reception this game has got, I think its good, and you have a lot of potential