This game has a great yet simple concept, it is a simplistic yet addictive game.... yet there are some issues..
The learning curve for example:
Game starts, a monster is on screen, and a timer begins rapidly counting down from 10 to 0. You are then given that amount of time to figure out that the game has already started, and that you need to quickly click on that monster... But that is not the problem, that's actually a good start.. sorta.. it makes you quickly snap to attention.
The problem is that right afterwards, you are put into the exact same situation, except there are around 10 of them. This increases the difficulty immensely, and with such a small hitbox, they're a little difficult to click on (but yet again, that's actually not that big of an issue).
This game would actually be a really good game, particularly for mobile devices, if anyone could make it past level 5.
To make a verbose message simple, it gets too difficult too fast.
As well, there should be a little more of an intro/title screen. The game begins too quickly, there should be a menu, rather than click to play, with options and a how to play screen, or even the simple instructions on the title screen.
All in all, a nice first attempt, it's really close, you're almost there, it just needs to be tweaked a little.