Or HaChaim on Numbers 24:18/אור החיים על במדבר כ״ד:י״ח

1 שעיר איביו, "even Se-ir his enemies." Bileam lumps Edom and Se-ir together as being one. The word ירשה implies that that which has been destroyed will not be rebuilt. Edom will not rise again from its ruins as will other nations. The reason is that these people were enemies of Israel, איביו. Compare Psalms 137,7 where the Jewish people are quoted as asking G'd to pay back the people of Edom for what they have done to them. The Psalmist pleads with G'd to destroy Edom to its very foundations. This is why Bileam predicts that Edom's defeat will be much more severe than that of the other descendants of Sheth.

2וישראל עשה חיל, "and Israel will do valiantly." Bileam means that the destruction of Edom and the descendants of Sheth will enable the Israelites to perform deeds of valor. Destruction of the Gentile nations and Samael liberates the sparks of sanctity trapped inside many of them. This in turn enables Israel to perform deeds of valor in "repatriating" these sparks of sanctity who had been stained while inside the bodies of the Gentiles. The expression עשה חיל suggests that one repairs something, rehabilitates it.