Although it's silly, you may be able to boil to death a red dragon
The distinctions both Dungeons and Dragons 3.5 and Pathfinder make between scalding damage and fire damage is, while sort of dumb, pretty clear: both sets of rules say, "Boiling water deals 1d6 points of scalding damage...," and scalding damage isn't fire damage. (However, I suspect many DMs and GMs would probably—no matter the letter of the rules—nonetheless say a red dragon's immune to damage from boiling water if serious business demanded it.)
However, a diligent chef should be aware that in Dungeons and Dragons 3.5 the spell boiling oil [conj] (Heroes of Battle 125-6) says, in part, that
A torrent of boiling oil rains down in the area you specify. Creatures in the area take 4d6 points of scalding (fire) damage from the oil. (126)
Thus it's possible that with this spell Noonan et al. were trying to put this particular rules quirk to rest by pointing out that scalding damage is, in fact, fire damage and would everyone just stop with the wisecracks already? But no one paid any mind, and the quirk persists. Whether it persists in the campaign is something best discovered through adventuring.
Likewise, in Pathfinder, in a thread where creative director James Jacobs will answer just about any question, there's the following exchange:
Question: Can you make a ruling about the effectiveness of cold/fire resistance and evironmental [sic] dangers of heat and cold? Does cold/fire resistance negate the dangers of cold/heat? Does it reduce the lethal damage one would take if the fail the associated save? Does one get a bonus to fortitude save for their resistance? Inquiring minds would like to know.
Answer: Damage from cold temperatures is cold damage. Damage from hot temperatures is fire damage.
Again, whether that's sufficient to change in a particular campaign the damage boiling water deals from the printed scalding damage to actual, for-reals, about-darn-time fire damage is best determined by experimenting.
Anyway, to aid you in your quest, here's a possible recipe.
Red Dragon Soup
This is the Dungeons and Dragons 3.5 version of this recipe. The Pathfinder version will likely require an oversized cauldron of brewing.
- Create a big enough pot to accommodate the red dragon by casting on the appropriate raw materials the spell fabricate or casting the spell major creation.
- Create enough water in the pot to immerse the red dragon by casting the spell create water.
- Entice the red dragon into the pot. Note: The author is not responsible for dangers encountered during this step.
- On the water in the pot cast the spell heat water.3
- Boil the dragon until dead. Note: The author is really, really not responsible for dangers encountered during this step.
- Carve dragon.4
- Serve in thick bowls with thinly sliced shriekers for dipping.
1 This step can be omitted if an appropriately sized pot is available, perhaps one affected by the spell shrink item.
2 This step can also be omitted if the (kind, generous, quirky?) DM allows the water in the pot to also be affected by the spell shrink item.
3 The 0-level Sor/Wiz spell heat water [evoc] (Dragon #302 50) targets only 2 pints of water, likely significantly less than what's needed. This chef suggests equipping sous chefs possessing ranks in the skill Use Magic Device with wands of heat water so that all of the water in the pot can be heated simultaneously.
4 This chef leaves this heavy lifting to a custom version of the spell blade barrier, but an adamatine weapon should work just as well.