I completely understand your critisism, and if I'd had just a single day more, the levels in question would probably have gotten easier. The difficulty of the levels was something I was very unsure about, since the idea was, that levels: 7, 12 and 18(I believe), should be kinda lika a mini-boss and introduce new game mechanics. I'm all for constructive critisism, and I completely understand, why you'd think that some levels are a bit(or a lot) frustrating.
I'm happy to hear that you enjoyed my game (except for those levels ;)) I'll probably release an updated version in a couple weeks time, since I'd really like, for people to experience the full game. The reason for it taking a couple weeks, is that I'm entering a period of many tests and exams, these next weeks, so I don't really know how much free time I'll get.
Thanks for playing my game, and I'll probably make my future games browser playable.