I am going to do an "Interactive" of the battle of Thermoplyae, your the commanding general of some three-hundred greek spartans, the most elite warriors know to man, about one-hundred and fifty thousand persians are heading your way to take your capital.
To the battle.
One of the best points at which to hold off an invader was at Thermopylae, a narrow valley adjacent to the sea. The attacker could not pass to the seaward side, and to go inland would mean a significant detour. Other armies could risk this, but Xerxes could not. Pressured by Persia he needed a victory and fast and decsive one. To begin with, the Greek army was surprised to see the Persians arrive so soon. They had hoped to get more reinforcements. On the other side, Xerxes had excellent information and knew that the Greeks were waiting for him. He set up camp on the plain below the pass. He was confident, but the army was so large that it could not afford to wait in any one place for very long.
A. Delay the land force and to defeat the Persians at sea, then starve the Persian army. By cutting of land supplies and blockading.
B. Send for reinforcements and let the persians gain precious time.
C. Call an all out retreat and make a stand with a much larger army and better equiped troops.
If you chose A. you are correct, you can delay the land force with bows and arrows until the mighty Greek Navy could defeat the Persian navy and starve the persian land force. If you chose B. your fate is grim and ill-mannered, the persians gain precious time and your men are overwhelemed. If you chose C. when you retreat the persians go out into an all out skrimish you and your capital is overwhelmed.
I may edit this later to have more options, if you do take the time to read this I am dearly sorry it is so long I would like to know how you think the small out numbered spartans held of numbers almost 100 times their size.