You are going down,man!

Five choice stones.
That's all he took. That's all, he felt, he needed. It was his weapon of choice. It was the weapon he felt 'comfortable' with, to defeat the champion of the opposing army. The stones, he knew, that came from the brook, were smooth and round and, therefore, could travel streamlined to their target.
Although the armies had been facing up to each other for months now, no headway could be found on either side. Now, the Israelites were facing an accomplished champion. He had championed many a battle for the enemy and had defeated many armies as their champion. Scarred and hardy, Goliath was a formidable foe, ready and skilled to face any opposing champion.
At this stage of the strategy of these armies, it seemed that champions facing each other could end this stand-off. But, the Israelis had not one champion to handle this decisive show-down. Well, they did, but King Saul was hiding and very willing to surrender his place to another by giving them his armor.
David,,however, had learned how to keep intended focus in battle while defending seemingly insignificant livestock.
David knew this strategy well, take down the main giant and see the host fall. Don't try and be the whole army, stay focused on your strength and your choice weapon. He was carrying a staff and maybe a sword, either way he was out to defeat this obstacle and reach the objective of the battle to defeat the enemy. He was going out with what he knew best - the sling.
The sling with 5 stones. If one doesn't work, there are 4 others that can and will do the job.

Even though mocked by the overwhelming odds against him in this battle, and by the foe himself, David went out with focused intention to bring this battle to an end. No hesitation. No half-hearted swinging of the sling-shot. David dug deep into his faith in the covenant they as a people had with the Almighty God and ran with surety and confidence toward the champion of the Philistine army.
This wasn't even a match in their minds. Like vultures ready to tear the flesh from the carcass, the Philistines hovered in the background, sure they were about to devour Israel and plunder their treasures of the battle field and who knows what else they intended.

What a surprise it must have been to them to see this young, small in stature shepherd boy, so skilled and intentional, deliver such a accurate, hard and well-timed blow to the head of this enormous warrior that it must have fractured his skull, after finding its way past his helmet armor. Needles to say, it brought the battle scarred and -experienced warrior to the ground and long enough for David to use the warriors own sword to sever his head from his body.
That day, that incident, that one strategic action defeated the hordes of Philisti. It also began a career for the young, now warrior David, as a soldier of note, general and noted champion. The Philistines continued to suffer casualties at his hand and experience the ever lingering reminder of their defeat that historic day on the battlefield.
One weapon of choice; one stone; one focused, intentional, passionate, young man; who could not be intimidated by the odds against him.