Basically the legacy is referring to his impact on Persia after his death, so what he left behind. Most textbooks you find design the information and organise his legacy typically under a chapter, such as 'an assessment of Xerxes' or 'an Evaluation of his life', in which you would also find a section about the interpretations of Xerxes. However, I wouldn't make your essay just about these representations, primarily because they aren't his impact. They are more so the evidence of his impact, how people depicted him in an Ancient versus a Modern Context demonstrates that he HAD an impact on society.
However, his legacy (things he is best remembered for) is things such as his building program (Persepolis indeed!), the fact that he provided a stable government (for 20 odd something years (can't remember actual number, my apologies), and you would also mention the expansion of the Empire that resulted from his reign. Also mention his religious policy, and his Foreign policy and the plethora of battles that resulted. You could also talk about the impact of his assassination, primarily on the Persian society at the time and generations thereafter.
I would mention the representations from Ancient and Modern contexts, though don't let it take the reign of your essay, as it is more so the evidence that demonstrate his legacies rather than his legacy itself. (if that makes any sense at all...)
