Thor has been striped of his powers, so must rely on his natural strength as seen midway through the Thor film.
Both Thor and Leonidas are fully bloodlusted, as Thor believes Leonidas is attacking Asgard and Leonidas believes Thor is attacking Sparta.
Thor has his battle armour (minus magical properties) and Mjolnir (minus magical properties).
Leonidas has his battle armour, a spear, a sword and a shield.
The fight occurs in an open desert.
Neither of them have prep time.
Both of them can rely on their years of battle experience.
Battle to the death.