How about Battle of Telamon, where the Insubri, Boii and Taurini fought to death even after seeing their Gaesatae allies perish or retreat from their positions?
It's true, that in general the Celts put much focus on the initial charge, but it's worth noting that the way of Celtic warfare was a lot different from what Rome and other organized states had become used to which were much more organized with clearer and often politically driven motives.
Most of the Celtic warfare were small scale clashes between neighbouring tribes' warrior groups, and just as much focus was put the superiority of invidual over his enemies (which exhibited in the head collecting practises, for example), the battles may have been intended to have been solved quickly - he who won the initial clash, established superiority over the opposing force. In smaller communities/populations, warfare has a more ritualistic nature and it's not unheard of that battles have been fought without a drop of blood being shed, as the value of invidual members of the tribe is greater than in nations whose populations are counted in the millions.
So, when Celtic warriors meet cohensive enemy units whose inviduals do not engage in one-on-one duels but rather push on as one solid mass, bashing with their shields and stabbing with their swords, it's quite possible that the Celts could have been at a loss as to how to defeat such a formation. Seemingly unstoppable, it's no wonder if they would've demoralized a tribal host (at least ones like the Celts' who fought primarily in mêlée and not in skirmisher formations).
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