I'm fine with dropping that altogether. It was just a brainstorm...nothing I was really attached to.
How's this?
Rejuvenation (Su): In most cases, it’s difficult to destroy a scorched one through simple combat: The “destroyed” scorched one will reform in 24 hours. Even the most powerful spells are usually only temporary solutions.
A scorched one that would otherwise be destroyed returns to its old haunts with a successful level check (1d20 + scorched one's HD) against DC x. The only way to completely destroy a scorched one is total immersion in water. Beheading a scorched one (either with a vorpal weapon or decapitation once it has been reduced to 0 hit points) renders it helpless until the next sunrise.
Do we need to retain the bolded bit?
For the vulnerability to water, we can modify this...
Vulnerability to Water and Wind (Ex): Ash mephits are particularly vulnerable to air effects and strong winds. An ash mephit takes half again as much (+50%) damage from spells or effects with the air descriptor, and are treated as if in gaseous form for purposes of wind effects. Additionally, ash mephits are particularly vulnerable to water, and contact with it (such as reaching into a pool or being splashed with a vial or bucket of water) inflicts 2d6 points of damage per strike. Complete immersion (including being caught in the rain or being subjected to a high volume of water, as from the geyser function of a decanter of endless water) inflicts 6d6 points of damage per round.
...to something like this...
Vulnerability to Water (Ex): Scorched ones are particularly vulnerable to water, and contact with it (such as reaching into a pool or being splashed with a vial or bucket of water) inflicts 2d6 points of damage per strike. Complete immersion (including being caught in the rain or being subjected to a high volume of water, as from the geyser function of a decanter of endless water) inflicts 6d6 points of damage per round.
Look OK?