Nightmare is there because some False Gods utilize shields which block physical damage completely, for example, Bone Wall (Morte, Incarnate), Dissipation Field (Chaos Lord), and Phase Shield (Gemini).
On the other hand, Long Sword is there because some False Gods chain Cloaks (Jezebel, Serket) or control no targetable creatures (Divine Glory, possibly Lionheart).
By the way, if you took enemy's HP down enough to finish him with Nightmare, then you could just wait until the effect of Shard of Sacrifice wears off.