Xena would win. She has killed superhuman characters,and Leonidas has lead an army.
Also,what a lot of people don't know about the last stand of the 300 Spartans was, that the Persian warriors,tho vast in numbers,were hardly trained in combat,had wicker shields,thin armor,no spears, and weak short swords.
While the Spartans are rigorously trained since they were young boys,essentially they were the Delta Force of their day.Also,the Spartans had strong bronze shields,strong swords,strong armor,and long bronze spears.So,Leonidas didn't fight any superhuman beings,he fought weak Persains.
So,with that comparison,Xena would win hands down with all of her dike-like moves.