Awful
The only good thing about this game is the concept itself.
The first few levels prove that fact, as I get to plan and execute an awesome 2D slaughter sequence.
The fun is quickly taken away, however. I gave up already at the first boss.
You see, there's something you screwed up.
The timing aspect, which is what this entire game revolves around.
The boss, I noticed, doesn't appear in a preset sequence of movement, but rather appears randomly in one of four corners.
This way, I could not predict where he would come from, and the whole point of the game was ruined completely.
It no longer had anything to do with timing, only pure luck.
Another thing that seriously annoys me is that you seemed to have missed something with the replay function as well.
"What?" you might ask now. "How could I possibly have screwed THAT up?"
Simple.
THE REPLAY IS NOT A REPLAY.
When I press 'Replay', I expect to see an exact duplicate of what had just happened. That, however, is not the case, because I get a slightly different ending every time.
For instance: While I missed a head in the first try, I can be lucky enough to get the headshot in the replay. Vice versa as well - If I hit the head in the first try, I can be unlucky enough to miss in the replay.
All in all, you fucked this up completely.
This game is not about careful planning or tactics or strategy whatsoever.
It's all about LUCK.
You might not have intended it to be this way, but I really doubt God intended humans to become atheists either.
Much unlike atheists, however, this game is awful.
Try again.
Fix the many bugs that I/others mentioned.