What's been working for me real nice for the Gauls is breaking my army into a left wing, center, and right wing sections with archers and general behind and a cav on each wing. I keep my front line straight until I get a close enough to the enemy then I move my right wing forward to attack the enemies left while continuing to advance my left and center. And then right before I proceeed to annhilate the enemy left wing I move my archers to my right side. For some reason, this freaks the AI out and it sends it's right and center to reinforce it's left wing. Then, I can usually flank these units with my left and center. And even if they engaged my left and center, that would work anyways since I want to prevent their wing from being reinforced. I find this pretty effective for me, but calvary heavy armies would worry me with this formation.

My army composition with this formation is 4 Hastati in the Center;, 5 principes on each wing with 3 units in front and 2 units in back; 1 equites on each wing; 3 archers, 1 general.

If I get triarii I might make my wings 3 principes in front and two triarii in back for better cavalry defence.