I try to avoid engaging forces I do not need fancy tactics to defeat. So in a regular fullstack vs fullstack with a heavy infantry based army(with no more than 3 Archers/slingers, up to 5 units of cav and at least 7 Heavy infantry) I try to form a solid line inside the forest(shock inf on the flanks). Then I start the battle, zoom in on the radar map and as soon as the enemy line is close enough to mine order my line infantry to engage the enemy as favorable as possible (guys with axes vs armored dudes etc).
Then I zoom in more on the mini map until units are shown as little dots rather than arrows, this helps keeping an overview over the battle. When the enemy is engaged I do simple Hammer and anvil tactics with my cavalry and freed up infantry. When the odds are not too unfavorable(I have levy civilized infantry while the enemy is a central european Uber stack^^) i usually win those battles.
General rule:
Gaul<civilized<otherbarbarian<Gaul