The rulebook does make it clear that the player's turns are very different when battling an ancient one. In particular, investigators may neither focus nor assist, and thematically, it could be argued that using special abilities during such a battle should fall into the same category. Prohibiting the use of special abilities when battling an ancient one would certainly fix the problem with Carolyn Fern. Furthermore, it wouldn't otherwise affect game balance too much as many other investigators have special abilities that can't be used during task completion anyway. Also, the ability to complete the combat task multiple times in the same turn would surely compensate for the loss of special abilities during these battles.