1)
What is the meaning of "bi'Zkano"?
1.
Targum Yonasan: B'Lo'ei. Radak - I did not grab only the hair, rather, with the lower jaw.
2)
Why did he say twice "I struck it"?
1.
Malbim: First I hit it, and saved the Seh from its mouth. 1 Afterwards it came against me, and I grabbed its beard and killed it.
1
Granted, here there was a need to fight for Hashem's honor. Also, surely Hashem will cause Galyus to die, like he deserves. Why did David risk his life to save the Seh? Perhaps Hashem put a Ru'ach Gevurah on David, and he did not sense that there is any danger at all. (PF)