I haven't heard much talk about the Ancient Dragon on these forums and I'm not sure if that's because people aren't fighting him, don't think it's worth it, or just think he's trivial and they're stomping him into the ground. Regardless, I found him pretty hard, and awesome. That said, I finally helped somebody beat him, and my experience crystallized a clear strategy in my mind.
First of all, use lightning. Lots of it. The more Great Lighting Spears you have to hurl at him, the better. I don't know of any other method of taking chunks of health off in groups of 400+ (don't be shy about sharing other serious damage-dealers). Use Sunlight Blade or Gold Pine Resin on your weapons if you're going melee.
This is the tricky part. If you can get under his hind legs he will try to stomp you with them. You need to be on the ball to evade effectively, but this attack does not make him change his positioning, so you can actually force him into a loop of stomping with one hind leg, then the other repeatedly by going from leg to leg. This gives other players a lot of time to throw lightning, or whatever else they're using against him.
While you're under there, he will occasionally stick his head under his front legs and breathe fire back at you. You need to get the fuck out of there at this point, but it also provides you with time to hit him with some ranged attacks once you're in the clear. I've yet to try it, but I'm willing to bet using Heavenly Thunder will be devastating at this point, since you're up relatively close, and he is a very large target.
Also, naked with stamina regenerating equipment seems to be the way to go, since he tends to one-shot you unless you've got the Gyrm Greatshield and lots of stamina.
This tactic seems to be pretty effective, but it's pretty important to have multiple players, with only one drawing aggro. What methods have you guys found to be effective? Please share.