With bad jokes, or having at least a pair of dwarves in the group, as dragons likes the taste of dwarven meat.
I think the best option is to use someone to endure the direct combat (plate armor, shield, supporting and healing spells) and the others using missile attacks and spells. It is a long long combat but at the end HPs will be depleted, or a character will have an open-ended attack after so many rolls with critical result for the dragon. If the dragon has AT 19-20, use lightning bolt spells, and why not shock bolt spells too for depleting some HPs. But the first spell you need to try are "pain" ones, those that reduces HPs by %, cast it again and again until it fails the RR or until its HPs are not good for it (then change to bolt spells), reducing 100-200 HPs at once can be hard (much resistance) but the reward is great if you achieve it.
The other way is using 2-handed weapons, but this requires all melee combatants have very good defense (plate armor with a good DB, good blessing spells, some bladeturn spells, etc.). This method is not very usefull if the dragon can fly.
In any way, IMO 1-handed melee weapons are not an option because their limited damage in proportion the big danger of stand in front or near of the dragon.
