Can't see what would be so hard about this style of game. What would you find difficult about tapping enemies to kill them? ;p
Of course, that would be a bit too easy, plus I think you'd lose the fun factor that comes from using a lightgun. Also, I think your hand / fingers would get in the way most of the time.
Just do a quick concept using boxes that appear randomly and you have to tap them to kill. Then you'll see if your fingers do get in the way or not.