Ranged Threat feat.
Iron Kingdoms longarm with an enchantment allowing it multiple shots.
Dragon attempts to disarm, shoot, crit.
Dragon attempts to fire the rifle. Quickdraw a pistol, shoot, crit.
I ended up doing somewhere in the vicinity of 200 damage with those two AoOs, which was sufficient to kill the injured dragon.
Don't ask me WHY the dragon in question tried to disarm my rifle, or why it tried to shoot it, or why the GM was allowing me to quickdraw and autofire, but that was my most memorable dragon-slaying.
In terms of serious suggestions:
Adventurers often try to attack dragons with the opposite element: cold, in the case of a red. Energy immunity (fire) is the first priority, whether from a wizsorc scroll or an item.
Adventurers also often try to attack from choke points where the dragon can't get to them except with its breath weapon, especially against a Huge or larger dragon (which can't squeeze into a 5 ft. gap). Don't leave these kinds of gaps!
Adventurers ALWAYS protect themselves from a dragon's breath weapon. So, any offensive spells the dragon memorizes should target other weaknesses. And sticking electricity or acid or sonic traps rather than fire traps would make them much more effective. Expect PC fighters to dive into lava without batting an eye when fighting a red. If the dragon has item-based cold immunity, use cold extensively, because it will be immune to that, to, whereas the adventurers won't be.
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