What are your cheapest core units to defeat Rome?

[Bodeni]

Bataroas have even less armour than Camillian Hastati, so even if you consider them "elite", they won't last in prolonged contact with even the lightest Roman line troops. Even the Golberi Curoas aren't equally armoured, though they just top out the Camillian Hastati in total defense.

But the charge bonuses! The lethality score! Slam a roman formation with these bad boys and watch them flee. Keltoi victories have to come quick or they won't come at all. Trick is to rout the romans as quickly as possible, Brihentin make good FM killers, Gaeroas and Gailache are best used to draw enemies into traps or as light reserves, Gallic elites are too pricey to use against anything but enemy elites. But Bataroas should be thrown into the enemy as soon as possible.
Keltoi don't do hammer and anvil, they use chisel and hammer instead.

[QuintusSertorius]

But the charge bonuses! The lethality score! Slam a roman formation with these bad boys and watch them flee. Keltoi victories have to come quick or they won't come at all. Trick is to rout the romans as quickly as possible, Brihentin make good FM killers, Gaeroas and Gailache are best used to draw enemies into traps or as light reserves, Gallic elites are too pricey to use against anything but enemy elites. But Bataroas should be thrown into the enemy as soon as possible.
Keltoi don't do hammer and anvil, they use chisel and hammer instead.

You don't get quick routs on Hard battle difficulty, there's a coded bonus to both defense and morale (and Romans already have high morale). So unless you're fighting a stack led by a captain, they'll be sticking around.

[Ca Putt]

When dealing with Romani there are imho just two vertitable courses of action:

Carrhae - Romani lack cavalry and don't have as many spearmen as most factions, so Heavy cavalry(or any lancer, cuepos, Illyrioi hippeis....) ideally supported by horsearchers.

Teutoburg forest - Romani sport large numbers of heavily armoured units but don't have a lot of archers, so getting lots of "weak" armour piercing units with nor armour can be rather usefull.

When in doubt add slingers and levy hoplites/Phalangites, but that's "Win EB 101".