I almost exclusively play as barbarian factions. In the early game, my armies are based on economics more than anything. The General and his heavy cav. are the backbone and centerpiece. I get rid of as many warbands as i can (exception: Germania needs at least 2 spear warbands in each main army). I rely on mobility. Combining 2 Generals with 1 or 2 barbarian cavalry in a mass and going for the enemy leadership is 90% effective for me. I use mercenaries to augment where i need to, especially Illyrian mercenaries, who are one of the most multi-purpose, effective units in the early game. I place mine on guard mode and use them for infantry as well as the traditional skirmisher/ambusher. I limit myself to one war dog unit, but in 1.2, the enemy uses them so i do too.

In the late middle game, i transition to chosen swords and chosen archers, but still mass my cav. to take out the opposing general as quick as i can.

In the barbarian regions, Heavy Chariots (Brittania) and Spear Warbands (Germania) are the greatest challenges. Mobility and wise use of terrain (especially forests) are the key to fighting with smaller forces for me.