The infantry tactics of the Roman army are well documented � most notably by uncle Caesar, or perhaps even Seutonius.
Against the fighting men of the legion however, Celts, Gauls and the Barbarian Hoards generally faced a daunting task. Individually, in single combat, the fierce warrior could probably more than hold his own against a Roman. In massed fighting formation however, the Celts rudimentary organisation and tactics fared poorly against the well oiled machinery that was the Legion.
The fierceness of the Celtic charges is often commented upon by some writers, and in certain circumstances they could overwhelm Roman lines. Nevertheless the in-depth Roman formation allowed adjustments to be made, and the continual application of forward pressure made long-term combat a hazardous proposition for any opposition.
Flank attacks were always possible, but the legion was flexible enough to pivot to meet this, either through sub-unit manoeuvre or through deployment of lines farther back. The cavalry screen on the flanks also added another layer of security. The Celts, Gauls and Germanics also fought with little or no armour, and with weaker shields, putting them at a disadvantage against the legion, and their logistics were poor compared to the detailed organisation of the Romans. There was a distinct lack of discipline amongst the Celts.
Archaeological evidence would exist in the forms of weaponry and armour which can be viewed at various museums throughout the UK (superior Roman Legion �vs- Celtic war band), and also in the documented evidence of various military campaigns which were helped along not least by internal feudal wars amongst British tribes.