Monks are actually surprisingly good with ranged weapons. High dexterity, high mobility, doesn't rely on shields for AC, can bonus dodge if necessary, wins archery duels by catching enemy arrows that hit. And a very common Monk race (Wild Elf) even has a bonus proficiency in longbows, so you get higher damage and longer range than shortbows. (This is about the only time when elven weapon proficiency seems to be any good at all.)
Monks are still inferior to a genuine Sharpshooter fighter with Archery style, but they are good enough at the ranged game that every monk should absolutely pack a bow. (Even the monks without proficiency in bows.)
So, for your specific party, I would say that the Monk, the Paladin, and the Barbarian should pack bows, and the cleric should cast Bless. (Cleric and wizard should pack crossbows.) That way you can engage from beyond his breath weapon range, which forces him to either escalate to closer-ranged attacks (breath weapon range or melee range) or to abandon the combat. In breath weapon range, your STR-based guys can switch to javelins and your wizard can throw spells like Web/Everard's Black Tentacles which target Dex. Alternately, your wizard can cast a Phantasmal Force illusion of, say, sticky webbing that brings the dragon crashing to the ground until he takes an action to disbelieve. I'm not sure what your cleric should be doing but then I don't really grok clerics beyond using them for Bless and Death Ward (which are actually better on a Bard).