What is the meaning of "va'Ta'anenah"?
Radak: This is like 1 "va'Ta'an Lahem Miryam" (Shemos 15:21).
I.e. this verb can apply to song. In Tehilim (119:172), Radak brings from Mesorah that in three places, Ta'an refer to praise. "Va'Ta'an Lahem Miryam" is among them, but our verse is not. (PF)
Why does it say "ha'Mesachkos"?
Malbim: These are not the women who sing plainly. Rather, they jest and say matters that can be understood in two ways, for jesting and Simchah. Refer to 18:7:4:1.
Why did they say "Sha'ul ba'Alafav v'David b'Rivevosav"?
Radak #1: Sha'ul killed thousands of Pelishtim, and David, myriads (tens of thousands).
Radak #2: When Sha'ul alone strikes Pelishtim, it is as if there are thousands [of Yisraelim] with him. When David alone strikes, it is as if there are myriads with him.
Refer to 18:7:4:1.
How could they praise David more than Sha'ul? This is like rebellion against the king!
Malbim: Really, the victory of Sha'ul and Yonason (14:13-22), when there were only 1,000 Yisraelim with them, was greater than that of David, with whom were myriads. Alternatively, b'Rivevosav refers to the stones 1 that he cast (this is like "Roveh Kashas" (Bereishis 21:20). However, Sha'ul understood simply, that Yisrael with David are greater than those with Sha'ul. 2