Need help in my shooting script

Hi community,
I'm new here, im working on somekind of 2d zombie defense, i 've script the player to shoot at mouse cusor, its work, but, my bullet is an arrow, i use transform.Lookat and the bullet some kind of facing to the mouse position on maincamera x,y axis, i know its some kind of weird , here is the picture to explan:


thanks for all the help
P/S: all i want is to make the arrow look exactly at my mouse cusor, on the player x,y axis, thanks alot, and btw, i have posted this on another box which is 2D box, but i havent get any answered....
and how to get more karma to post question on unity answer? i know if we have 50 karma so we can post question, if not, we must answer or vote for karma, but i dont know anything, how can i answer to the question i even dont know what he is asking about???
Thanks for all your answer, its help me alot.