Either by the lightnings, or thunderbolts, or other things which accompanied the cracks of thunder; or by the Israelites, who perceiving them to be affrighted and flee away, pursued and smote them, as the next verse mentions.
Related Commentaries of 1 Samuel 7:10
1 Samuel 7:10
10 And as Samuel was offering up the burnt offering, the Philistines drew near to battle against Israel: but the LORD thundered with a great thunder on that day upon the Philistines, and discomfited them; and they were smitten before Israel.