Record of Lodoss War
In the anime Record of Lodoss War, there were three dragon fights shown totally (there is a fourth, but it only shows scenes from it while a narrator describes it). In the fight that Invader is describing, even the axe hit only did minor damage. What finally did the dragon in was the thief throwing a dagger into its eye, causing the head to jerk downward, allowing the main fighter to drive his sword through the bottom of the dragon's skin into its skull. The dragon then convulsed until the cavern collapsed, crushing it.
The second and third fight consisted mainly of an Everquest type of fight with a dragon. They would have one character get the dragon's attention, where they would jump and dodge the attacks, losing hit points without actually "getting hit", while the other fighters would get above the dragon and leap on top of them driving magical spears or swords into their necks or heads.
You could always use the "hard on the top, soft underbelly on the bottom" belief, where the PCs could be hacking up its underbelly, and when they do enough damage the dragon's insides will poor out, thus killing the dragon without really having to describe how a 3 foot blade is able to piece to the heart or other such nonsense.
Or, use the hit points the same way as you do with the abstract system for characters. Once the hitpoints of the dragon are reduced, think up some way to have the dragon die that would be feasible, such as how Wulfgar and Drizzt kill the Great Wyrm white dragon in The Crystal Shard, there just happens to be a large stalactite hanging right above the dragon, and wulfgar throws his hammer at it, causing it to fall and scewer the dragon.
I understand that you might have trouble coming up with multiple unique ideas on doing this, but unless you have a party of dragonslayers, how often do you have the PCs fighting gargantuan or colossal dragons during a campaign?
Just a few ideas.
