Why did he say "Pen Yomar Oyvi Yechaltiv"?
Malbim: This corresponds to "Ad Anah Yarum Oyvi Alai."
What is the meaning of "Yechaltiv"?
Radak: I overcame him, and he would not rise again from under me.
Is there a difference between a Tzar and an Oyev?
Malbim: Yes. A Tzar pains in practice. An Oyev seeks his evil, but does not afflict in practice. The verse teaches a Chidush. Even though Hashem hid His face from him, he is given over to afflicters, to the point that the enemy, even though he normally does not fight him in practice, just seeks his evil covertly, will say 'I overcame him.' It is as if seeking his evil sufficed to make him fall; he did not need to fight him at all. The Tzar, who normally fights, sees that he falters by himself via Hashem's Hashgachah.