You're complaining about what two infantry squads and a flame tank can do?
What happens when you use units in their intended roles is this: they destroy disproportionate amounts of the enemy in terms of cost.
What I've often done in this situation is to mob them with armor just after a preparatory barrage.
One option might be to give artillery a bonus when firing into forests. During WWI and WWII forests were known as deathtraps because the tree trunks would cause airbursts and the forest would become filled with jagged splinters flying at high speed.
I really don't see this as being all that much of a problem though. Flame tanks are indeed LOLWUT but then flamethrowers have been barbecuing soldiers for a hundred years now.
Am I wrong in thinking that there is a rather stringent limit on how many flametanks are available? For NATO it's 4 Zippos if I remember right. That's hardly enough to complain about. Am I really that misguided?
on others it is vital that you control the forests.
If that's really the case then you should probably control the forests. Get there firstest with the mostest.