We edited the Casse a bit for MP but the general idea remains. You use eagles and druids to keep your forces from breaking while you use fear units (chariots, druids, uridusios) to quickly break some weaker units and start chain routs. Casse battles are often among the shortest we have in MP. They do really well against the Romans oddly enough (since Romans don't kill quickly and usually rely on morale as well) while they do poorly against factions like the other Gauls, especially the Aedui (who can do almost everything the Casse can with heavier suits of armor).