Belzbet, it seems to be taking your fighters 5 rounds of constant firing to kill the dragon. That's enough time for two breath weapons assuming the dragon opened with one. I'm not so sure the fighters would win.
For example:
Dragon breaths on people, and advanced into melee with the party I believe your suggested composition was 2 dwarves and two elves? I'm not sure on their exact stats, so feel free to correct this scenario if I'm off.
Party shoots the dragon. You have two elves using longbows and two dwarves using slings. Let's say the elves hit it, dealing 9 damage. Dragon is at HP 13.
Breath weapon time. Everyone takes 11 damage, on average, on a failed save. +2 or +3 reflex vs DC 15 reflex save means that's a distinct possibility. That's most of a fighter's HP... it's probably going to knock out any elves in the group unless they put a 16 in Con. If this happens, the party's down by two members.
The remaining dwarf fighters, say, of Con 16, have 2 HP each. They can try to drink potions, which means the dragon gets to full attack them with +5 bite and 2 +0 claws, or initiate grapple, a touch attack.
This is not good. This is really not good.
Assuming everyone succeeds on their reflex save and takes 5 damage, the party is now in melee with a dragon. And they are armed with ranged weapons. This is also not good.
It gets kind of complex from here, as the party can continue to shoot at it, take a round to switch weapons, take a round to drink healing potions... or maybe the dragon decided that, since he failed to soften up the party, he would fly away until his breath weapon comes back and he can try to wipe out half the party again. It can burrow, after all, so hiding from the archers is probably a valid tactic.
I personally would not be too optimistic.
But assuming the fighters do win... well, there's also some monsters that would kill a fighter only party but would be easily defeated by spellcasters.
Bat Swarms, for example.