What is the meaning of "ha'Kel Me'azreni Chayil"?
Radak #1: He strengthens me.
Radak #2 (Shmuel II, 22:33): When I was in the Chayil (army), He was my strength.
Malbim: This is one of the two ways how Hashem saves Tzadikim from mishaps. Even though Hashem gives to him strength, speed and teaches to him war strategy (verses 34-35), he does not fight or run. He stands in his place and the victory comes by itself, miraculously. 1
"V'David b'Rivevosav" (Shmuel I, 18:7) implies that David actually fought and killed! Below, Malbim explains that sometimes David actually fought. Refer to 18:36:1:1.
What is the meaning of "va'Yiten Tamim Darki"?
Rashi: He cleared my path from any pitfall, sin or danger. It was complete and smooth.
Radak: He makes my way complete; not one of my soldiers was lost in war.
Malbim: I did not need to fight or harm anyone. Refer to 18:33:1:2.