Now I have played your game, beaten it and I can say Congratulations... This game really can be considered as complete considering that you will not add anything else to it and it is considered to be a collection of single levels rather than a certain storyline. I found no fatal errors and found no too annoying bugs. A lot of levels are very well planned and designed and some of those even look like a level from Battle Toads. Less amount of those levels but still a lot of those was that kind of levels that really fits with my personal taste, especially boss fight levels. Boss fight levels have that much effort by you so I even still want to replay those. Those are difficult but worth of tension.
Those looks like a bit like Power Plant from IGP but you also added much more different effects which are very good.
There are some things that I personally don't like that much. For example, water enemies who act like lava balls that looks like a bit bugged. They jump out from the water and may jump on the bridge where from they never jump back to the water. I am sure those enemies can be improved so they would act like lava balls but who would be killable also.
Tar or water pieces unconnected in the air even if those come from the walls are really not my thing. I know some games have something like this too and one of those are games like SMW but you may want to add special graphics for those tiles so those would look fictional rather than glitchy.
Boss from Greenland is really small and I wanted to know if you don't want to use your own sprites for your bosses. Maybe you wanted to make this boss look small and it is the only boss which is really easy so I can agree with the size but how about 32x50 instead of 32x32 or 50x50?
Those are some of my very small examples which maybe you want to improve.
Long explanation about the cons does not matter.
I just wanted to be clear but those are very small things that maybe can be called as okay also. But here is one bigger thing... Your game has become to be one of the best games on Explore now but the ending is just a boring built-in ending page. You don't have to use complicated systems everywhere otherwise but don't you want to include a small piece of scene-cut and ending credits to your game?