battling in forests needs even more micromanaging than horse archer armies. if you are the ambushers it is good news just encircle the enemy and decimate them but if you are ambushed just order your important general run away. I learned why it is so important to have one unit of scout army after I ordered one of my consular army to siege a celtic town, and ambushed, my consule KIA..

other than ambushes normal forest battles gives you advantage on lots of things , archers and cavalry sucks in forests that is why no nomads managed to rule germany with the exception of Attila.

and if you are defender in fact less probable.... let the enemy tire their soldiers.
use and prefer javeliners who are better than archers in forests.

if you are attacker the most common scenario, if you have archers just use them to reveal hidden units as they have no use other than it.

if the situation getting worse use emergency orders : defending triangle / square, cavalry charge at enemy's back... and pray of course....