Really solid first submission! Congratulations :)
I especially appreciated the idea of the timed acid rain, and the levels were clear and well designed. Sometimes, certain enemies were unnecessary (like the jigsaw over the flames during an acid storm :P), but that's not a big deal. The timer, for me, was the biggest enemy - in the sense that it was perceived as the main danger. Just my experience, but I hope that it helps for the future :)
Great job!