Albert Barnes Commentary Job 41:28

"The arrow cannot make him flee: Sling-stones are turned with him into stubble." — Job 41:28 (ASV)

The arrow - In Hebrew, “the son of the bow.” As in Lamentations 3:13 (margin). This use of the word “son” is common in the Scriptures and in all Oriental poetry.

Sling-stones - The sling was used early in war and in hunting, and by skill and practice, it could be employed in such a way as to be a formidable weapon; see (Judges 20:16; 1 Samuel 17:40, 1 Samuel 17:49). As one of the weapons of attack on a foe, it is mentioned here, though there is no evidence that the sling was ever actually used in endeavoring to destroy the crocodile. The meaning is that all the common weapons used by men in attacking an enemy had no effect on him.

Are turned with him into stubble - Produce no more effect on him than throwing stubble at him would.