Achilles destroys Leonidas in this fight. He is superior in every way.
1) He is better throwing spears. Achilles can throw a spear further and more accurate than Leonidas(this is proven, Achilles throws is spear further and hits his target perfectly, Leonidas throws a much shorter distance and misses.
2) Achilles is skilled at fighting with a spear at close combat. We can't be sure how good Leonidas is at close range with a spear.
3) Both men fight a huge man in the movies. Now there is no denying the giant Leonidas fights is tougher than the giant muscle man at the beginning of Troy, we can all agree that they are both big, strong men. The difference is Leonidas would of lost to his without help, Achilles kills his man with one blow, after just waking up and having sex all night.
4)I have many more points, but i'll end it with just one more. Leonidas, himself says that without the man next to him, he is as good as dead. They are trained to fight as a team. This fight is one on one. Achilles specialty. He goes country to country killing their best warriors one on one, its what he does and he doesn't even get touched. The best warrior he's every faced(Hector) was only able to scrape his armor.
Clearly Achilles wins this 9/10