Physical prowess was not enough

Goliath began his training as a soldier when David was born. David’s age (16-19 years) was the years of experience as a soldier for Goliath. David had not even tested or practised with the armour that soldiers used at that point of time. (I Samuel 17: 39)
Challenge: With his imposing physique, strength, stamina, and thundering voice, he was undefeatable giant. Goliath the Philistine was six cubits and a span, meaning 9 feet and 9 inches. Some translation gives the height as 6 feet and nine inches. He was well armed with latest military armament of that time. Bronze helmet, coat of mail, sword, spear like a weaver’s beam with about seven kg iron head, shield, javelin and bronze armour for his legs. Arrogant Goliath challenged the Nation of Israel to provide one combatant to fight against him. The winner’s victory will be that of the nation.
Obvious response: The obvious choice to fight Goliath was Saul. It is recorded that Saul was a head taller than anyone in all Israel. (1 Samuel 9:2) That means he was 6 feet tall. However, Saul was trying to recruit someone to fight Goliath.
David volunteers: Saul was not impressed with David. However, David insisted that he had different experiences with wild animals and was successful. Reluctantly, Saul agrees, but provides full armament to cover David, which is not comfortable. Saul tried to make David a prisoner of tradition. Instead, David chose a sling and five smooth stones. (I Samuel 17:40)
Unprotected part: Goliath ridiculed David. He however, went to war in the name of the Lord. (I Samuel 17:45) Goliath’s forehead was not obviously protected, as was observed by David. Stone is obviously faster than javelin or spear and could be thrown from a longer distance. In fact, the longer the distance, the greater the impact. Bronze helmet and shield could not stop the fast-arriving stone from a sling. Certainly, Goliath did not expect a surprise attack with a stone.
Do I have zeal for the Lord, faith in the Lord and wisdom from the Lord as David had?