You want a spell that makes a unit fire at a certain point for a duration?
1. Do you want the spell to do AoE damage at the area it hits? Then you can base it of, say Rain of Fire. Change the character animation from "cast, spell" (or whatever it says) to "attack". Change/remove particle effects at target area.
To make an actual missile moving to that location, i think you'll have to use triggers.
or
2. Do you want it to damage one/all enemies in the line beeing shot? Because then you need triggers.