1)
Why did he say "Siger Elokim"?
1.
Malbim: Surely this is Hashgachah that all are sleeping, so he can strike [Sha'ul].
2)
Why did he say "ba'Chanis uva'Aretz"?
1.
Radak: I will strike him greatly, so the spear will [go through Sha'ul] and be inserted in the ground.
3)
What is the meaning of "v'Lo Eshneh Lo"?
1.
Radak: I will not need to strike him again. The first blow will kill him. Malbim - therefore, you need not fear lest he awaken due to the blow, and scream.