Why did he say "va'Shem Osi k'Gibor Aritz"?
Radak #1: They intend to kill me. I do not fear this, for Hashem told me that they cannot overcome me; He is with me like an invincible Gibor.
Radak #2: He made me like an invincible Gibor to stand against them.
Radak citing Targum Yonasan: Hashem's word is with me like an invincible Gibor.
Malbim: Hashem is my Os (sign) and flag. He is illustrated on my banner as an invincible Gibor who fights to save me.
What is the meaning of "Lo Hiskilu"?
Rashi: They did not succeed with their counsel.
Radak: They will be ashamed, for their actions and intents for me will not succeed. This is like "David l'Chol Derachav Maskil" (Shmuel I, 18:14). The past is used in place of the future.
Why will their shame be forever?
Radak: People will always curse them.
Malbim: They did a base act, to slander me and testify falsely.