This card feels far too narrow and win-more to me. Already have enough creatures in play that this deals substantial damage each turn? Then you are already winning and are probably better off playing a lock piece. Few creatures in play? Then this card does nothing and you'd much rather spend two mana on a Ravager, etc.
I think the most exciting applications are as a combo piece in some kind of token or elves deck; that still sounds very fringe, though. If only it read, "whenever one of your creatures becomes tapped, opponents lose one life..."