mortar is on to sumthn here w the dummy unit idea imo. you can group the unit along a trajectory i guess and spawn a dummy unit for each and make the dummy attack those units. them using fan of knives might work out ok.
if you're going to the left, get the unitx of the trigger unit and minus a certain amount, and check for units within a small range, then minus another amount, and do it again. just make sure don't overlap and let units get hit multiple times. in fact you probably don't even need dummy units for this unless you need them for graphics...should be able to make the triggerunit damage the units to the left of him.
going to the right, get the unit x and add a certain amount...
going up, get the unit y and add a certain amount, going down, get the unit y and minus a certain amount...combine these mthods logically in order to shoot in 8 directions
i recommend...using replacement method.
make 3 abilities
these could also be items of course, or one being item and two being ability, or vice versa.
2 abilities rotate the fire zone. so if you're facing up(90 degrees by map making rekoning)...press buton left twice then middle button once to fire. then it takes another 4 buttons either left or right to shoot right again (0 degrees by map making reckoning)
the idea is that, everytime you press the rotate left or rotate right abilities, the shoot ability changes. so when you click on it, it shoots in a different direction.
another option of course, is to say screw all that...and just make one ability that detects the facing angle of the triggerunit and shoot the weapon in that direction...but it didn't soundlike that was what you were going for.
i think you wanted to press one button to shoot in one direction, not 2-5 as my solution presents...but you perhaps also did not want to rotate the unit by clicking on the ground as i understood it. those are the only two things i can think of. consider which is the greater evil. dota traditionally uses the latter method. whatever direction the hero is facing...the projectile goes there. of course....it's an easy matter to point and click(after one hotkey button) and then the hero could should in that direction...but it sounded like you didn't want that either.