David overcame the Philistine with a sling and a stone! You must be joking! A man with no armour but just a sling and a stone facing a giant (3m tall) with full armour & spear in hand?! David knows that it is not he who could do it but God. He killed a lion and a bear before and acknowledged that it was the Lord who has deliverd him. Not that David has extraordinary strength and undefeatable. If he is, I'm sure he would never have to flee from his enemies as he alone would be able to bring a whole army down! However, at God's chosen time, place and reason, David was able to gain the victory because God is with him. Something can be impossible for David but nothing is impossible for God!