So how to be a Dragon Slayer
Here are a few thoughts.
Use a Wizard
. Transfix or Compel could be very effective against a Dragon. Even with the Dragon's Magic Resistance, the Dragon will probably have to use up its precious Will store eventually. If it's successfully transfixed or compelled, it can be swarmed by several of your troops since it has such a large base. If its Will is gone it will be more likely to flee if it is actually wounded.
How can the Dragon counter a Wizard? The Dragon can toast the Wizard with firebreath before the Wizard forces the Dragon to use up its Will. Or the Dragon can use Wyrmtongue to Sap the Wizard's Will and then Transfix or Compel the Wizard. Furthermore, the Dragon can use Fly to stay out of the Wizard's range and then fly into range to breath fire or cast a spell or charge the Wizard.
Ok, another idea. Use Archers
--lots of Archers. Elves with Bows could be effective since they are accurate and their S3 bows wound Def 7 on a 6. If they are able to shoot enough times, odds are that they will be able to cause some wounds. I think Def 9 would decrease the probability by 50%. Legolas and other Elven heroes with Bows can be especially dangerous since they can modify the wounding roll with Might.
Dwarven S3 bows have the same probability to wound but the shorter range could be a problem if the Dragon can fly. The Grey Company could also be effective because they can have so many accurate archers. However, their S2 bows will have a significantly harder time wounding D7 or D9. At least the Grey Company usually has a lot of bow-armed heroes with Might to help with wounding.
What can a Dragon do against Archers? There are too many variables like terrain, scenario objectives, supporting units, etc. to cover everything. However, as Beowulf
described, getting the Dragon into combat quickly will keep it from being targeted by archers. Flying would be useful to attack quickly or fly out of range or fly into cover if necessary. Tough Hide would obviously decrease the odds of being wounded. Firebreathing would only be useful if the archers are clustered together. If the Dragon can't fly, compelling an enemy forward with Wyrmtongue could get the Dragon into combat quicker.
Just some food for thought...
OK, back to work