Well, SoP buffs immortal creatures. We know from Spark that 0 hp creatures can survive using SoP.
Thus, this would be too OP with SoP, unless you somehow got in a specific clause to make SoP not work, like an ability called "Impatient" or something.
Also, if you think about it, this is basically the same thing as a spell that does physical damage. This means that it's weaker, since the usage of Dim Shield, DR Shields, Dusk Shields, and Fog Shields eclipses the usage of Reflective and Emerald Shield. Therefore, if you wanted to balance this, you'd probably make it a bit cheaper than a spell that deals the same damage.