Feel free to list the exact composition of units needed to take out a Doom Fortress when you don't include artillery.
Feel free to list the exact composition of units in a Doom Fortress when you don't include flame tanks.
Artillery is what I recommended for clearing them. They are annoying but surely not such a huge deal.
What I think is that this is unimaginative or poor play on the part of the people involved. One thing you can do is to charge with several tanks, with one on point. As soon as the point tank makes contact, pull it back. This will use up a targeting cycle on the part of the flame tank, while your other tanks will finish theirs and get a shot off.
I'm finding it rather hard to understand why people can't come up with a counter to this. Numerous counters have already been mentioned, from unit counters to tactical counters to maneuver counters.
Here's yet another for you: sacrificial recon vehicle with artillery in very close support. Fire as soon as you see the flame tank, with your corrected barrage circle centered somewhat behind the flame tank. His likely plan is to fire, then pull back, and if he does that he'll end up zeroed.
People always complain about this sort of thing and I never really understood that, especially in this game where there are so many counters available including a basically identical unit. You are somewhat unlikely to be able to storm a defense without losing units, and that's just the nature of war, and since Wargame is nominally realistic it's the nature of the game as well.
They'll probably nerf it to some degree. Until then my best suggestion, at the risk of sounding flippant, is this.
Deal with it.