I dont know, it sounds complicated
Its a fireball that needs to:
Respawn
follow a path
explode and damage the player
respawn again
repeat
Sounds to me like a coding endeavor, Especially for the part where it will hit the player and damage them.
Fortress files idea would work, but you might not be in the right area for it to look as if the fireball hit you, as that trigger would be there all the time.
although, thinking about Fortressfiles idea, You could, use a func train, and time the activation of the trigger_hurt so that it only works when the fireball is within a small window of oppurtunity to be hit, then couple the trigger hurt with an env explode, and env render to disappear it. But again, a limitation here is that the player has to be in a thin hallway, or a thin walkway, and the fireball moving horizontally across it, to acheive a realistic effect.
Ill tell ya what, if I get home from school today and your still having trouble, I'll make an example map with what I mean.