i dont kow how this trigger works but id make it this way (there might eb a simpler, easyer way)
Create a unit that has 9999999999999999 range or w/e, range doesnt matter and give it around 100 area of effect ground target. Make it have an attack cooldown equal to the intervals of the fire and give it a modified burning oilthat does 999999999 dmg. burn oil must last as lnog as fire. make a trigger that makes your unit attack as soon as fire sprouts. I hope this helps and again, there probably is a simpler way.