I don't know what triggers her to use the skill.
I placed her on the west where Fallens spawn (Nazeru's Gates Iron challenge) and she used it only twice on them.
Anyway, she excels in crowd control on desert stages more than any other ground-based heroes.
I tested dante, nivus, alric, cronan, bruxa on Buccaneer's Den campaign with the following layout (1 assassins and 1 dwaarp) on the west and didn't use rain of fire.
Only Bruxa could manage it with no leaked enemies.