On the day David faced Goliath in battle, the giant was not his most dangerous enemy.  David faced and overcame several unexpected enemies on his way to defeating Goliath, and they posed a far greater threat to his success that day:

• Fear
• Criticism
• Doubt and discouragement
• Compromise

On his way to the battlefield, David overcame fear by turning his focus away from Goliath and onto his Deliverer.  David overcame criticism by ignoring it and walking away from his accuser.  David overcame doubt and discouragement by remembering what God had already brought him through. David overcame compromise by staying true to his convictions and shunning the ways of the world...and finally, David slayed the giant.

Additional scripture references: Romans 8:28, Proverbs 23:9, Proverbs 26:4, Proverbs 18:2