Datto has a great little video analyzing how Weapons of Light, Shadow Shot & Melting Point stack. Basically it boils down to Shadow Shot & Melting Point do not stack, in fact the weaker Shadow Show enemy debuff will override Melting Point's enemy debuff. However both stack fine with Weapons of Light. So: Don't use Shadow Shot & Melting Point at the same time, it's a waste. Do you Weapons of Light, it always helps.