What did Uncle Leo mean when he said: "she can only kill me"? (sixth grade can really kill you)

He is being sarcastic, he is basically undermining the fact that death is a fate that means the end. So essentially he is being ironic with his sarcasm

one thousand and one nights, also known as arabian nights is a collection of stories whose basic structure is that of embedded narrative. this technique refers to a main story which works as the common frame - or framing narrative - for the other stories, and then this series of stories which are connected to the main one. this technique can also be referred to as a story within a story. the stories stem from the original tale, some begin and end on their own, some are connected to the previous or following one, by means of questions that are answered in the subsequent tale, or characters which reappear.