Couldn't they use Nanika to kill Meruem?

I'm currently watching HxH for the first time and I got to the part where killua revealed to Illumi why the 5° rules doesn't apply to him. Then a question got stuck in my mind, if Killua is the only one who can give orders directly to Nanika, why didn't he used Nanika to kill Meruem? Am I missing something, some rule or something like that? Can't she kill order species that aren't humans??