This might be of interest: from Livy, Book 38, describing Cn. Manlius Vulso's expedition against the Galatians ("Gauls") in Anatolia. The original source in Polybios.

[38.19]Subsequently, more definite information was received from Oroanda to the effect that the Tolostobogii had actually occupied Olympus; that the Tectosagi going in a different direction had established themselves on another mountain called Magaba, and that the Trocmi had left their wives and children in the care of the Tectosagi and gone to the assistance of the Tolostobogii. The chiefs of these three tribes were Ortiagon, Comboiomarus and Gaulotus. Their main reason for adopting this mode of warfare was that by holding the principal heights in the country, provided with everything they might require for an indefinite period, they hoped to wear out the enemy. They never imagined he would venture to approach them over such steep and difficult ground; if he did make the attempt they believed that even a small force would be sufficient to dislodge him or throw him back in confusion; whilst if he remained inactive at the foot of the mountain he would be unable to endure the cold and hunger. Though the height of their position was itself a protection, they drew a trench and constructed other defences round the peaks on which they were established. Missile weapons they troubled themselves very little about as they thought the rocky ground would supply them with plenty of stones.

[38.20]As the consul had anticipated that the fighting would not be at close quarters but would involve an attack upon positions from a distance, he accumulated a large quantity of javelins, light infantry spears, arrows and leaden balls and small stones suitable for hurling from slings. Provided with these missile weapons, he marched towards Olympus and encamped about four miles' distance from the mountain. On the morrow he sent Attalus with 500 cavalry to reconnoitre the ground and the situation of the Gaulish camp. While thus engaged a body of hostile cavalry, twice as large as his own force, sallied from their camp and put him to flight; some of his men were killed and several wounded. The next day the consul went out with the whole of his cavalry to explore, and as none of the enemy appeared outside their lines he made the circuit of the mountain in safety. He noticed that towards the south the ground rose in gentle slopes and was covered with soil; on the north the cliffs were precipitous and almost vertical. There were only three possible roads - everywhere else it was inaccessible - the one up the middle of the mountain free from rocks, and two which were difficult, one on the south-east and the other on the north-west. After making these observations he encamped for the day close to the foot of the mountain. The following day, after he had offered the sacrifices and the first victims had given favourable omens, he advanced against the enemy. The army was formed into three divisions; the largest he commanded in person and began the ascent where it afforded the easiest approach; his brother, L. Manlius, was ordered to advance from the southeastern side as far as the ground allowed of his doing so safely, but if he came to a dangerous or precipitous part he was not to struggle against the difficulties of the path nor try to force his way over insuperable obstacles. In that case he was to turn and march across the face of the mountain and unite his division with the one which the consul was leading. C. Helvius was to work gradually round the lower slopes of the mountain and then take his division up the north-eastern side. Attalus' auxiliaries were also formed into three divisions, Attalus himself accompanying the consul. The cavalry and elephants were left on the level ground at the bottom, and their commanders were under orders to watch carefully the progress of the action and render prompt assistance wherever it was required.

[38.21]The Gauls feeling confident that on two sides they were unassailable directed their attention to the southern slope. To close all access on this side they sent 4000 men to seize a height which commanded the road, distant rather less than a mile from their camp, where, as in a fort, they might prevent the enemy's advance. When they saw this, the Romans made ready for battle. Somewhat in front of the legions went the velites, the Cretan archers and slingers and the Tralli and Thracians under Attalus. The heavy infantry advanced slowly as the ground was steep and they held their shields in front of them, not because they expected a hand-to-hand contest, but simply to avoid the missiles. With the discharge of missiles the battle began, and at first it was fought on even terms as the Gauls had the advantage of their position, the Romans that of the variety and abundance of their missile weapons. As the struggle went on, however, it became anything but equal; the shields of the Gauls though long were not broad enough to cover their bodies, and being flat also afforded poor protection. Moreover, they had no weapons but their swords, and as they could not come to close quarters these were useless. They tried to make use of stones, but as they had not got any ready, they had to use what each man in his hurry and confusion could lay hands on, and unaccustomed as they were to these weapons, they could not make them more effective by either skill or strength. On all sides they were being hit by the arrows and leaden bullets and javelins which they were powerless to ward off; blinded by rage and fear they did not see what they were to do, and they found themselves engaged in the kind of fighting for which they were least fitted. In close fighting where they can receive and inflict wounds in turn, their fury stimulates their courage; so when they are being wounded by missiles flung from a distance by an unseen foe and there is no one against whom they can make a blind rush, they dash recklessly against their own comrades like wild beasts that have been speared. Their practice of always fighting naked makes their wounds more visible, and their bodies are white and fleshy as they never strip except in battle. Consequently more blood flowed from them, the open gashes appeared more horrible, and the whiteness of their bodies showed up the stain of the dark blood. Open wounds, however, do not trouble them much. Sometimes, where it is a surface bruise rather than a deep wound, they cut the skin, and even think that in this way they win greater glory in battle. But when the head of an arrow has gone in or a leaden bullet buried itself and it tortures them with what looks like a slight wound and defies all their efforts to get rid of it, they fling themselves on the ground in shame and fury at so small an injury threatening to prove fatal. So they were lying about everywhere, and some who rushed down on their enemy were being pierced with missiles from all sides; those who got to close quarters the velites slew with their swords. These soldiers carry a shield three feet long, javelins in their right hand for use at a distance and a Spanish sword in their belts. When they have to fight at close quarters they transfer the javelins to their left hands and draw their swords. Few of the Gauls now survived, and when they found themselves worsted by the light infantry and the legions coming on, they fled in disorder back to their camp, which was full of tumult and panic, as the women and children and other noncombatants were all crowded there together. The Romans took possession of the heights from which the enemy had fled.

Polybios, Livy's source, is one of the best historians from the ancient world— both as a historian of political interaction (between superpower and local powers), and as a military historian: for instance, the famous narratives of Hannibal's battle of annihilation at Cannae (216) all go back to Polybios. In this particular instance, the tactical narrative is very full (notably full of practical details for future commanders that might be reading-- part of the utilitarian bent of Polybios' histories), and also full of detail, because fighting Celts is specifically described as one of the things that statesmen should know about (in case the "Celtic peril" should strike Greece again). Here, a particular striking bit of narrative: enemy seizes commanding hight, gets dislodged specifically by concentrated "firepower", and final charge cut down by sword-armed light infantry.

In this particular passage, Livy probably lays it on with a trowel in description of wounds, the occasion for a "psychologizing battle scene" that was a favoured genre morceau in ancient historiography (carnage as the occasion to portray the effect on people), and also to pile on the ethnographic cliches (Gauls are barbarians without self-control and endurance, unlike civilized Romans). I'd very much like to know if the info. about sling bullets burying themselves in flesh is Polybios, or Livian overlay.

Polybios came from the Peloponnese-- i.e. the Achaian League, and the original Achaians (N. Peloponnese) were famed slingers (notably the inventors of the kestros, the dart-sling, which he also describes)

Note that British celts (a little later) are famed slingers. Yes, Polybios and Livy like to make their Gauls irrational, in contrast to rational, calculating civilized people (Greeks, Romans). On the other hand, this is our only source, and Polybios was very careful about his battle narratives.

For me still a certain doubt remains.  I suppose that even in ancient times stone throwing wasn't any more effective against javelins, slings and arrows. So why the Gauls should have relied on throwing stones? They should have known that fact by experience. Though their practice seems to be very irrational as Livy describes it, this may be rather a result of a lack of long range specialists or just a foolish tactical mistake on that day (as this happens often enough in history). But you are right. Livy is the only source, so this issue will never be definitely claryfied.

funda

Note that the Athenian assault onto Psyttaleia, at the battle of Salamis, starts with a hail of hand-throwns (Plut Aristeides): the marines (heavy infantry), leaping off the ships onto a shingle beach, throw stones at the small Persian force occupying the island: this gives them enough time and space to form up and charge

Note also that Vegetius, in his famous passage on infantry skills, says that recruits should learn to throw stones by hand and by sling (this is the passage where he says "whirl once, not three times").