No one on the Spartan team, Leonidas included , is beating Achilles mano'e'mano, but everyone on the Spartan team was akin to their leader in skill and strength. The Myrmidons have much more fodder potential. But if they can thin the numbers at least a little, Id think Achilles can handle a fair few Spartans on his own. And due to Inverse Ninja Law if it's him outnumbered on the home stretch, seeing how the Spartans don't use bows, I say he swings it.
That's all assuming the respective leaders are present, Spartan soldiers vs myrmidon soldiers? Spartans win.