They're heavily drugged with something akin to PCP. A good missile hailing as they approach should weaken them well enough. Hit them with cavalry from the flank if you can when they hit your infantry line; it'll cut them quick, and it's what Romans did at Telamon. Their morale and staying power is intentionally very good, but a wise tactician who realizes what the way is to fight them can break them pretty quick. They are not supermen, but they do require special tactics.
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