Born of the Three Thunders - understanding the feat

[Darrin]

It has to be acid, cold, fire, electric, or sonic damage, so I think that invalidates Scalding Mud.

Nope. It has the [fire] descriptor and does fire damage. I think it has the best damage output of all the "Kill It With Lava" spells, even though it doesn't actually use lava.

Elemental Darts has the same problem as Scalding Mud.

Not really, as it has the [electricity] descriptor and it does electrical damage.