I've noticed something: survival seems to = "kill sh*t quickly."
When I mixed Earth and Air Magic on Kathkit, she didn't kill sh*t quickly, and even though she had Armor of Earth up almost constantly, she died a lot. Turned her into a Fire Ele, she killed sh*t more quickly, she died less often.
Karla's a Minion Master, but stuff seems to get around the minions quite a bit (either that or there aren't any minions because nothing's died yet, anything that's died didn't leave an exploitable corpse, or they've all died because there was a loooong distance between new minions-in-the-making... or some combo thereof), so she either dies because she can't kill sh*t quickly or her minions (if she has any) can't kill sh*t quickly enough (i.e., before they die).
Touka, though she has more armor than casters, can't kill sh*t quickly, so she seems to die only slightly less than EarthAir!Kathkit.
I've learned from my mistakes with Kathkit, so Sally's now a Fire Ele too and she doesn't die as much, either.