Hint: Don't fight in them.There was a reason the Roman Empire had a dreadful time in Germania. A good strategist will ensure that he doesn't need to fight his battles any where near those damned forests, because the cost of such a battle is so high. Take a different route, or use a forward force of light infantry to cause them to attack, then retreat that force back to the main one. Usually the AI will follow and you will be fighting on an open battlefield.

Remember to use right-click on the strat map to get an idea of the terrain you would be fighting in.

Foot