How many giants have you killed? If the answer is "none," ask yourself another question: "How many giants have you fought?" Usually the answer to the second question is the same as the first. We've killed no giants because we haven't had the faith, guts, and love to fight any.
Every man in the Israelite army refused to fight the giant Goliath. "Then David spoke to Saul: 'Let your majesty not lose courage. I am at your service to go and fight this Philistine.' But Saul answered David, 'You cannot go up against this Philistine and fight with him' " (1 Sm 17:32-33). If we volunteer to fight a giant, all the cowards will put us down in order to justify their cowardice.
However, our response should not be focused on defending ourselves, accusing the cowards, or sizing up the giants. We must focus on the Lord to be giant-killers. Thus, David's words in response to Saul were: "The Lord...will also keep me safe" (1 Sm 17:37). Also, when the giant Goliath insulted, cursed, and threatened him, David responded: "I come against you in the name of the Lord" (1 Sm 17:45). David ended his speech by proclaiming: "The battle belongs to the Lord" (1 Sm 17:47, our transl).
To kill a giant, you must fight one. To fight one, you must not focus on the giant, yourself, or anyone else. Fix your eyes on the Lord (see Heb 12:2).