Neat flash game. I do like the catapult mechanic a bit. The thought of using tangent angles did really help. But that somewhat took a fair amount of difficulty out even in the harder mode. I made it to levels 10 easy and 8 on hard before I decided to stop. But there's some fun to be had for sure! And plenty of potential.
My favorite parts are when there's lots of collisions, like from the dragon or when a rock combos into things. Thing is that I wish there were more ways to accomplish that and there were rewards for it. Like maybe there's an enemy with a bomb strapped to themselves and explodes like the dragon, though because of their consistent position you'd want to direct hit them first instead of dropping them. Maybe there's a triple shot power up on the field you could go for. Etc. Aside from that some more variety in stage hazards would be good as well. Maybe unbreakable platforms, bumpers to bounce off of, moving platforms, so on.
But otherwise it is fun enough and the mechanics themselves are tight. I'll note that there's a bug with the catapult where if you LMB while the catapult is in motion for a throw, it'll throw from where you clicked. And I find the most annoying thing to be when the catapult changes directions when you try to go for max charge. Those are my only gameplay problems. Otherwise for a boss rush game there's a lack of bosses as others have pointed out. Overall decent game but leaves me wanting to see more out of its potential.