Originally I had planned to add explosion too. But halfway through I lost the motivation. But thanks to your comments. I felt like I should add it. I have updated the game now.
Also, the control originally was mouse click. But some people suggested that clicking mouse rapidly causes some hand-pain. So I had changed it to space bar. But I think different people have different opinions. So now I have allowed both the keys to function exactly the same. The game title is "One Click Rumble" anyways. So it seems reasonable to have mouse click as an fire option too.
For the boss level and tougher enemies - I think I'll do that after this game, because right now the code is such a mess. :-P
I want to first refactor everything so that it becomes easier to add more features.
Mobile version: I have a android version exported too. But after getting it tested by some of my friends, I found out that it probably makes sense to have two hand control for mobile version. One to move the player and other to fire. But as I said, this project right now is too much messy. But I'll definitely try to release an android version of this game.
Anyways, thanks for playing this game. I love constructive feedback for that anything. It really helps me in improving my future projects too. :-)