I guess you're play VH battle difficulty? That's always crazy, since the Ist Cohort, which is already very powerful, gets +7 (?) bonuses for everything, or something like that. Playing medium battles is always a solution, completely fair to both sides.
Anyway, you could always wait for them to approach you, and attempt to rout their weakest unit with pila volleys. Then, get your cavalry to kill the enemy general, further weakening their morale. The rest was explained above, swamping and flanking. They should eventually break. If not, just play another campaign with MEDIUM BATTLE DIFFICULTY!
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