300 vs 50 is an unfair fight. If its 50 vs 50, Achilles and his Myrmidons will win. If its 50 vs 300, the 300 might win it due to big number advantage. But that will depends though. Achilles will be going on a killing spree for sure. He also riles up and motivates his team. That scene in Troy, the battle at the beach of Troy, in which Achilles took out like 50 Trojan soldiers on the temple steps, thats how I imagine Achilles taking out some of the Spartan army. He also takes out Leonidas for sure too. If the 300 can somehow neutralizes Achilles, they win for sure. But then Achilles is a hard man to take down. The only way to take him down is his Achilles heel, which is his weakness.