Those who had camped in the hills would be the Edomites and Ammonites of 7.18.

Took to flight: Good News Translation‘s “began to retreat” fits in well with the beginning of the Israelite attack in the next sentence.

Then the men of Israel … rushed out upon them: When the Assyrians were in general retreat, then the attack began. This could be made even clearer; for example, the whole verse may be translated “The soldiers who had camped on the hills around Bethulia began to retreat, and at that moment all the Israelite soldiers came charging down on them” or “At the very moment that the Assyrian soldiers who were camped on the hills around Bethulia began to retreat, all the Israelite soldiers came charging down on them.”

Quoted with permission from Bullard, Roger A. and Hatton, Howard A. A Handbook on Judith. (UBS Helps for Translators). New York: UBS, 2001. For this and other handbooks for translators see here.