hey there, thanks for the feedback. The screen size might be because of the device and the shooting is from left mouse but you dont have to hold it, instead click on an individual enemy the amount of times it says on the screen. For eg: if it says 2 clicks to kill an enemy. You have to click (left mouse click) 2 times on the enemy in order to kill them.
I hope this clears it, any more doubts you can ask me :D