Stop thinking like an hellenistic general and all that "hammer and anvil stuff".
Replace your current bodyguards with elite infantry, like sweboz and greeks (A MUST WITH CASSE).
...
KILL'EM ALL!!!
1) family members proudly in the first line (guard mode works wonderfully against non elites and save your stamina)
2) swordsmen in the second line: SAVE THEM AS MUCH AS YOU CAN, high lethality swords and no armor means that even if you win you'll suffer huge casualties.
3)LOTS LOTS LOTS of slingers (3-10): barbarians fall down like flies against missiles and after gaining a little experience, nothing can stop them

4) keep the rabble inside cities: you don't need meatshields, UNTIL YOU FACE HELLENISTIC AND EASTERN CAVALRY

5) place near the frontline some frightening guys.
6) recruit a little cavalry only for pursuing: YOU ARE NOT ALEXANDER, if you like surrounding a lot, go east
Celts are great fun, keep on fighting!

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