you should not ban a creature, at least try to comes with a solution, there is always at least one
For example against iron dragon, check all spells wich ask for a MR save. An example : If you play Jotunheim, have your mages cast disintegrate, with a spell focus for further MR breaching. If you compare what you must invest compared to what cost a dragon (20 gems ok, but researching to level 9 in construction too), then you will see that it is not as desequilibrated at you might think.
Check also spells which hamper move, like the various vine, entangle and bond spells (nature, fire, air, ...). They will stop in their track a creature for one round. In the meantime, some units can hammer at them.
There is also blindness, which will reduce the attack and defence too zero. Ok attack wont be important, because they trample (so I dont think the stat is used), but having a defence of 0 will mean that your troops will be able to kill them rather easily.
Just some ideas, but there is more.