IMO, you're not really supposed to kill a Great Drake, unless it's part of a plot. If it is, the GM could provide a special item etc to make the task possible (but not easy). If your GM allows you to kill Great Drakes just for fun, he/she'd have to invent the Ye Olde Grand Drake afterwords. Or dump a Behemoth in a desert or mountain range nearby.
Mega-monsters like these are meant to keep the players (and their characters) on their toes, so although killing one shouldn't be completely impossible, if there was a fail-safe slayer's manual on how to kill one, the need for a more dangerous monster would present itself soon.
Or suddenly dragons should begin to travel in packs.