So, let us begin by listing out the few key differences between tanks and walkers, so we can then know what to engineer our plants to attack.
- Tanks are lower, making them more stable, with a low center of gravity
- Tanks are easier to armor, because it is simply a box, with fairly low surface area compared to a walker
- Tanks can carry much heavier guns, also because it is stabler
Well, this should be quite easy to attack, you can simply have the fauna constantly spewing out some form of thick, denser-than-air gas. This gas would then play havoc with the tank's short stature, and the crew would then be unable to see anything. And that is with the gas simply being there, you could easily than also make it toxic, corrosive, or radioactive or something else.
Thickly armored tanks
Tanks are at the very simple concept, they are simply METAL BAWKSES, with plenty of important things on the inside, such as ammo, crew and the engine. In real life, an armor piercing shell hitting a tank only creates a fairly, relatively tiny hole, but such a hole is enough to stop a tank dead, as the shrapnel would then fly about on the inside and hollow it out. So, now you create plants with tentacles, coated in super acid. Let this super acid cut through anything your people can create, so heavy armor tanks do nothing against that. Meanwhile, walkers, which also tend to have quite a number of important things in a small compartment, have the advantage of being more agile, as well as being more "holey", as for the same amount of weight as a tank, they tend to be bigger. Not only that, an acid penetration to the arm of a walker simply means no more arm, not no more walker.
Tanks have big guns
Well, this should be quite easy to attack. The forest can be also extremely overgrown, meaning that it is not easy to maneuver large and heavy weapons around. Another thing to note might be that the super acid tentacles may be fairly fragile, and can be easily destroyed with mild gunfire, so submachine guns akimbo may be more effective at fending off acid tentacles than a tank cannon.
Well, that's all the stuff I can think of now, I am sure I will be able to add more later