Really cool animation! In the thumbnail there was just a white square so I had to click on it and have a look. Then when the animation started my jaw dropped a little. Just the design of the first charecter looked so nice and then the action sequence that followed. Really cool.
I am not that into anime so I don't know if the transformation into cannons is novel or not. Heard about girls that turn into tanks but this seems more dragonballish to me. Regardless I love the concept and the animations of the guns and explosions are great. The moves the main charecter does to transform are also great and I love how his helmet melts away to reveal his face. Really nice work. Would love to see it in color.
It's hard to fault something like this bccause it exceeds my own abilities by so much but after viewing the animation a few times the only thing I can mention is that the charecter's face in the end looks a bit wonky. I think there is something with the eyes? Perhaps the spacing is too narrow or something. Also, the buildings in the explosion scene are a bit uneaven, which is fine but there is one or two that look thinner at the bottom than on top which gives a strange feel.
Overall though this is amazing work and my comments are minor details that I am more than sure you know already and might even fix in the future. If you decide to do fully rendered version in color, please share! :)