If you can't avoid fighting Gaul in the forest (which puts you at a high disadvantage) then all you can hope to do is group your army into a few groups and organise it in flanks so you can remember where each flank is by number - or just scroll down and watch your whole army underneath the threes. It's a lot harder, so I would agree that you should just withdraw off any maps which FORCE you to fight in the Forests.... use the campaign map to work for you