Did Elazar strike them by himself?
Radak: He overpowered in this war more than the other two Giborim, therefore it says that he struck them.
Malbim: He began, and Shamah ben Age finished (verse 11). In Divrei ha'Yamim I, 11:13 it is concise, so it attributes the victory only to Elazar, for he began.
Why is it written Yashvu, and we pronounce it Yashuvu?
Radak: They had fled from the Pelishtim - "va'Ya'alu Ish Yisrael" (verse 9). When they saw the strength of Elazar striking Pelishtim, they returned after him.
What is the meaning of "Ach Lefashet"?
Rashi citing Targum Yonasan: To strip the corpses. No one helped him to kill them, only to strip the Mesim after he killed them.
