What is "Shamir Chazak [mi'Tzor]"?
Rashi: Shamir is a worm. They show it [where they want to engrave] on a stone, and it cleaves under it. Malbim - even though it itself is weak, via a Segulah it is stronger than rock, and can cleave Chalamish (flint) rock. So I made your forehead - you will defeat them via a Divine Segulah.
Comment in Rashi #1: Shamir is a hard rock. In Targum Yerushalmi, it is the Targum of Tzur. Also the Targum [Yonason] of "mi'Tzur ha'Chalamish" (Devarim 8:15) is mi'Shmir Tinara.
Comment in Rashi #2: Shamir is strong iron. Just like a hard rock is called Shamir, also iron, due to its strength.
Radak: There is a Cholem in mi'Tzor, like in "va'Tikach Tziporah Tzor" (Shemos 4:2). Hard iron is called Shamir. Just like iron is stronger than rock, and cuts it, so you will be broken and ashamed.
Why does it mention two kinds of fear?
Malbim: You will be fear their evil, and you will not feel lowly in front of them, because they are many and honored.
Why does it mention "Ki Beis Meri Hemah"?
Radak: I know that they will stand against you when you rebuke them.
Malbim: They will be Batel opposite you.
