Two ways then to possibly do it.
1. Evaluate the AOE around the unit with the phoenix fire ability with a pick every unit. If the unit has buff "phoenix fire", draw a line of lightning between the phoenix fire unit and the unit with the buff.
2. There is a trigger somewhere that has something that refers to "source of damage". Perhaps look for what triggers that and evaluate whether or not source of damage would point to your phoenix fired unit (then draw lightning)
Are you sure that this doesn't generate attacked events? I know it generates death events because I evaluate when things die to phoenix fire enabled units in my map.