Who pursued whom?
Rashi (according to R. Nechemyah), Rashbam and Targum Yonasan: The enemy pursued a thousand of Yisrael, and two, ten thousand... .'
Hadar Zekenim (Vayikra 26:8, based on Mo'ed Katan 16b), Da'as Zekenim (in Vayikra 26:3) and Rosh (in Vayikra 26:8, citing the Ramban): Yisrael chased the enemy 1 (with reference to Adino ha'Etzni [David ha'Melech)], killing eight hundred in one go 2 , and not a thousand, as he did earlier [due to his sin with Uriyah - Hadar Zekenim]). 3
Riva (Vayikra 26:8, citing R"M of Kutzi): Yisrael chased a thousand of the nations in the days of Mashi'ach, and two, ten thousand.
Da'as Zekenim and Rosh (citing the Ramban): They should have pondered how one person - Shimshon, could chase [and kill] a thousand Plishtim, and two - Yonason and his armor-bearer, chase ten thousand, if not for the fact that Hashem did all this?
See Shmuel 2 23:8.
Kol Eliyahu: Rashi learns this from the fact that Maseches Sofrim 8:1 listing the differences between David's Shirah in Shmuel II (22:1-51) and in Tehilim (18:1-51), states that, whereas in Shmuel (which was written first) it says in Pasuk 44 "Tishmereni" (Gematriya a thousand), in Tehilim, it says "Tesimeni" (Gematriya eight hundred).
What is the meaning of "Im Lo ki Tzuram Mecharam ... "?
Rashi: 'If their Rock (Hashem) had not sold them out 1 and Hashem had not handed them over to us (finished them off - Targum Yonasan)'. 2
Hadar Zekenim: "Tzuram" refers to Avraham. At the B'ris Bein ha'Besarim, when Hashem showed him a smoky oven and a fire, to hint at subjugation under the nations, and at Gehinom, one of which his descendents were destined to suffer, and asked him to choose one of them - he chose the latter. 3
What are the implications of "va'Hashem Hisgiram"?
Oznayim la'Torah: Refer to 32:30:2:2. It refers to the fact that it was Hashem who advised Avraham to choose Shibud Malchiyos. 1
Because, the Yisurim in Gehinam are infinitely more painful than the Yisurim in this world (as the Chachamimsaid 'the fire os Gehinom is sixty times as hot as the fire in his world) which explains why the Torah uses the term "Hashem", which denotes Midas Rachamim. See also Oznayim la'Torah.
QUESTIONS ON RASHI
Rashi writes that one enemy will chase a thousand of Yisrael, and two will chase ten thousand. Regarding B'rachah, the Torah writes in Vayikra 26:8 that five of Yisrael will pursue a hundred, and a hundred will pursue ten thousand. If so, the Midah of punishment exceeds the Midah of good, contrary to what the Gemara states in Sotah?
Refer to Vayikra 26:8:151:1-6 and note.
Oznayim la'Torah: The change of Lashon from "Echad Yirdof Echad Elef" to "Shenayim Yanisu Revavah" implies that the Torah is referring to two different issues - that one Nochri will chase after a hundred fleeing men of Yisrael, and that two Nochrim will cause ten thousand attacking men of Yisrael to flee.
