Right, I have a new proposal for this thread:
Stop thinking like Romans and start thinking like orcs.
Working against the environment is all fine and nice, but it rarely works out too well for anyone IRL or in fantasy. With the notable exception of recent Iraqi work against the marsh Arabs, draining swamps is the only way to take out a 5,000 year old way of life.
The Orc thing to do is to adapt the environment or create new environments.
Minor tactic: Don't take out the trees, replace them! You have a whole continent of horrible species to work with, the elves just have their silly wood. Send out some brilliant supply officers and begin seeding the nasty parasites. Throw up organic bulwarks of your own. Nasty vines and things that the elves won't recognize as your work or your threats.
Remember the Orc thing to do is not to develop counters to the enemies tactics but to change your own so that the enemy has weirdness to work with. Elves are long lived and rely on an advantage of prior learning. You're always learning everything for the first time, turn that against them.
Major tactic: Rivers?! Roads?! An Orc craves not these things. Orcs like swamps and Orcs LOVE tunnels. Don't move around in the woods through nasty arrow infested areas, stay underneath them. Build Barracks that are everywhere and nowhere all at once. Build floodable tunnels that can cause the ground to go swamp when you need it to. Kill roots, don't kill trees.
And remember, always remember, that you are better at night then elves. They still need light. You don't need nothing. Go for the small spaces, the enclosed spaces. Take the fight to the trees. You're stronger than they are and if anyones gonna brachiate it dang sure isn't going to be a pansy elf.
If the trees are speaking to the elves, that's brilliant! You know where all their spies are, start feeding them misinformation.
Also, their using bows in a forest! Sure they can snipe but, they've already lost the range and massed fire that's their major advantage. You're spear chucking, club throwing infantry you're the way forest soldiers were meant to work. Start going German.
Move in dispersed formations through nasty sight reducing terrain, remember cover is your friend more than theirs, and develop real moral reducing battle language. Horrible ulating cries that echo through the woods. Let the woods whisper, you shall howl!!! You are orc. Make certain that when your unit uses it you all use it at once. Confuse them, keep yourself fine, remember it's the only language most of your troops know.
March divided and fight united. Loosing one or two troops to snipers is nothing. Loosing a troop to an ambush is the horror. Coming upon an ambush as you march off the path and then turning on it is the tastiest of dishes. When you camp. Do it underground, in concentrated groups, and make your tunnel systems much larger than they need to be. Let the dire animals dig to find you. And when they do let them find themselves in horrible narrow tunnels with orcish spears on either end. And then there is meat.
And yes, tunnel under and around the keep. Starve them, but also make them bleed. Sortie into the castle. You can afford to send in a suicide mission every night. Can they afford to stop them?
Don't burn the trees, just fill the air with smoke.
Don't assault the walls, bleed them. You are the leech and elves are the ill humour. Don't show yourselves to the enemy. Stay in your orc holes and wait. Let them sorty into your vine filled woods and your trap filled tunnels and your superior forces, and let them die.
Make certain that your supplies are of as little value to the elves as possible and try to make them actively harmful. Trap them. Fill your holds with horrible undead. They won't eat the food, and when poor elfy lets them out
Use q ships at every opportunity. You need less supply then they do, but you have more supply to give. You know they are going to succesfully ambush you from time to time so embrace it and make them choke on their own success.
Get compasses if you can and make certain that your troops know how to navigate from fixed points and can judge their distance travelled perfectly. If the forest lies to you all the time then don't talk to it.
You are Orc!
You are the swamp that gives beneath the feet. You are the tunnel that worms through the might of mountains. Trip them, trap them, choke them, and make them drown in the blackest blood.
I absolutely agree that you must remember your Art of War, but remember not the lesson that you should be resolute, but the lesson that you should be formless.