The concept is very solid along with the art, but the execution is lacking. First off, a reaction game should have a random start, not a count down (like in a race where they have a countdown and then a random start). Also the place where you are aiming each round should also be random. It doesn't make sense to have more than one bullet and also the computer doesn't seem like it will miss after round 5 or so but I didn't test it.
For reference I got killed in duel 8 after 0.5 seconds. If the average human reaction time is ~0.25 seconds then you are giving the player ~0.25 (0.5-0.25) seconds to move the mouse across the screen. But since the start isn't random you are really giving 0.5 seconds and I couldn't do that even as an experienced FPS player :(
Tbh it was terrifying as the duels went up if that is what you are going for XD
But touch screens will have a major advantage.