there's no way to damage every unit within the line of the projectil's flight without triggers..
so you basically could do two methods (that came in my mind):
>> Use the shockwave spell, and everytime the unit want to attack, order it to cast the shockwave instead, replace the model with an arrow or whatever you want to use, and modify the ability of course.. and it will work, plus the cool terrain effect that the spell normally has xD
>> trigger the whole stuff yourself.. like creating a dummy, and order it to move to attacked unit, and give it immolation or stuff like that, change it's model to an arrow and remove collisioon size.. blablabla.. would be some more work, i guess..
There isn't a way to do this without triggers, maybe you could set the Range to 150 oder something like that and change it to "missile (line)" and use like 99 units he can hit, maybe it will work, but I don't think so, so there's no trigger-free way, as far as I know.