the celts didn't stand a chance, although they were brave.
There are precious few examples of Celts (or Germans) beating the Romans in a set battle. Once the Romans were lined up a wise barbarian commander would have led his army off the field. The "warrior" code which pervaded those armies prevented that action, of course.
The Romans did not fight as "warriors." They fought as a team. The legionaries forming a bristling steel wall while auxiliary archers and their scorpions and ballista fired over the heads of the infantry to weaken the depth of an enemy line. It is not generally known that by the 1st century AD the Romans had perfected the technique of relieving men in battle.
A soldier in the front rank would fight for five minutes or so and then be replaced by the man behind him. The tired soldier would go to the rear of the line and rest, working his way slowly forward until it was his turn to fight again. Incredibly effective technique and significantly morale-boosting.
Of course, when strung out on the march they were susceptible to ambush, particularly if the commander was an idiot and failed to scout ahead. When not in proper formation the Romans were at a disadvantage and the Celts, Germans, and even Judeans obtained some notable....if temporary successes against them.