I think guild wars 2 wyvern's a good example of a tough but fair fight. May I post that here and point out some stuff from it?
I'll post it. the main things are is that while airborne it is way faster than on ground. Makes it very hard to predict and react to which is good.
It's fire is special and persists for a long time when it hits a surface. It burns so hot it kills players in only a few seconds. Every time in video you see a blue arrow it's a player that died. It happens a lot! See 5:00 in video for example.
Also it's wings are so big it can use them to AOE air blast players and knock them away.
ZOS - The Dragons have 14M health points but for the most part are just sitting on the ground taking damage from the surrounding players. The outcome is inevitable. If it isn't too late to change your programming, I suggest that they fly around more while distributing their own damage to players. Have them swoop down on players forcing quick evasion. The closer a player is the more damage is dealt to the dragon and vice versa. Something along these lines to force weapon swap, movement, and more of a challenge tactically.
And make the rewards bigger for killing them. For example - 3000 Gold, 3 soul gems, a rare crafting item, or anything that will draw groups to come back again and again and again.
I think it can be as simple as making the flying attacks faster and harder to predict, and making the fire on ground last longer and burn hotter. Those 3 changes might be very helpful in keeping the fight exciting.
I found the dragon fight to be really unimpressive. The beast barely moves, it's like fighting a statue that twitches occasionally. And it only seemed to have two major attacks. Having a 'half-digested backpack' to hold the loot was cute, but the loot itself was lousy (ordinary shock bow, some Magicka potions and a Soul Gem IIRC).
In short, it needs a lot more work given that this is supposed to be one of the selling points of the Chapter.