Adding +1 damage for each attack

[FannyShaver]

Well, it's already done and to avoid abuse it's set as a damage source due to the fact that only two units will be attacked.

 

[D359]

Easy way

USe the passive Incinerate spell to do this,make it a unit spell,and set the damage to +1 per target,this should help increase damage done by 1,to a target,but since this is spell is for a tower in a TD I recomend you add a spell that stuns the mobs and bloodlust to attack faster.

Incinerate Arrow is the autocast version of the spell I suggested you use.The incinerate spell can be found in the object editor,at the cattegory spells for neutral hostile heroes.