1)
What is the meaning of "va'Ya'as Chayil"?
1.
Radak citing Targum Yonasan: He gathered [Malbim - many] soldiers.
2)
When did he strike Amalek?
1.
Radak: This was after he was commanded "Lech v'Hikisah Es Amalek" (Shmuel 1:15:3).
3)
Why does it say "va'Yatzel"?
1.
Malbim: He intended only to save Yisrael from their afflicter, but not to wipe out Amalek's name due to the past, like Hashem commanded.