You can detect the basic attack of a unit (this can be quite complicated, but luckily, you have a tutorial on it xD). You can then make the unit attack instant, instead of missile. When the attack is detected, you can order a dummy unit to send a breath of fire-based spell (if you make the Area of Effect and Final Area the same, it will be a line, not a cone). Or use a missle system, but a dummy caster should be the simplest solution. You can use the new DAMAGING event in order to change the attack type to the attack type of the unit and set the damage type to normal.