Here is a relatively simple solution for this problem.
2 cheap infantry ~ 40Total generalized cost
1 recon infantry ~ 40
2 mortars ~ 60
1 cheap heli (preferably with grenade launcher/rockets) ~ 30
This requires a good bit of micromanaging, but that makes sense since this is a tough spot ingame to deal with. Take your recon infantry and set them on hold fire. Then,For large forests:
Slowly move them through a forest flank until they spot an enemy unit.For small forest patch:
Hit an accessible corner with the mortars and push 1 cheap infantry in. If they survive at the edge of the forest, move in the recon unit and slide it forward until you spot an enemy unit.
Pull your infantry group back and take note of that enemy position. Hit it with the two mortars. Wait a few moments to see if a flame tank moves up; if so, move in your overhead cheap heli to stun it, then hit the flame tank with your infantry and mortars. Repeat this process to remove the enemy presence. If the enemy masses in the forest and attempts to remove your small attacking force, that's even better. They will be consolidated into a nice little spot for your mortars as you attempt to pull back your ground and aerial forces.
I have worked this tactic a few times, but it doesn't seem too necessary for the vast majority of games I play. Usually,
I just blanket non-essential forests with artillery for a while as I skirt far around them. And as others have stated above, there are indeed a multitude of ways to counter a heavily defended forest. It all just comes down to what's going to be most effective for the price.
Sure, you could rush the forest with swarms of infantry or splinter it with megatons of explosives, but those don't have a very good cost/benefit ratio. Pure heavy artillery does, but without large accuracy and reconnaissance they're not the ideal solution. Plus, with a solution like this, you can actually gain ground in that forest and in turn make it a headache for your opponent.