The word "Herculean" in this excerpt from The Apology is an example of a literary device called allusion.
An allusion is used to make an indirect reference to a significant event or a famous character: in this case, Hercules and his strength. In that sense, "Herculean", which refers to somebody that has great strength and courage, derives from the name of the celebrated hero Hercules and it functions as a literary device that allows the writer to simplify a complex idea.
