If you can't outgun them, outrun them. Archer Auxiliaries are good troops, but they can't carry a battle all by themselves. I've never seen the Romans bring many archers, they tend to be very melee infantry heavy. But if they do bring archers, concentrate on bringing more melee troops, preferably cavalry (or, in the case of Britons, chariots). You might lose some troops against the Arc-Aux, but after all is said and done, chariots are a decisive weapon, while all kinds of missile troops are more the "nice to have" support kind of thing. Besides, isn't facing powerful Romans the point of playing a barbarian game?
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