Healing skill as a out of combat healing using MP potion in exhchange, resting is quite different as it doen't recover MP.
The frequency for Fairy using "Prank" skill is too low, I wait for more than 3,40 round in total and it still not trigger.
Viewing post in Balance issues and feedback
I looked into the game's code (sorry). It seemed like a Fairy uses Prank only if it's damaged (sorry again if it's incorrect).
Prank has 3 different (seemingly random) outcomes, but in my playthrough I got Frog TF a few times in a row, then Cow TF a few times in a row. Maybe some mechanic to reduce this behavior of randomness could help. The easiest I can think of is to rotate between those outcomes (e.g. Frog, then Cow, then Pig, then Frog again).