Against masses of infantry, the fear factor is king. Archers with flaming arrows, bring some scary infantry, chariots (yes, you read right) and send a volley of javelins in the backs of your enemies followed by overwhelming charge of 3-1 local numerical advantage (preferably including cavalry). They will rout and you can move on to the next part of the line. A lot of your Gallic troops are good in forests too, and IMO you dont need slingers against sweboz, so while its probably better to avoid the chaos of forest battles, dont be scared of fighting them, you can win.

Your line troops just need to be any kind of javelin equipped troops with guard mode activated.