When Shankles are frozen after revealing their weak point, they won't move after hiding it again until the next time they show their weak spot, even after unfreezing. This makes them much less of a threat (at least for some time) when battling without having to kill them.
Should this be added to the strategy section of the Shankle page? (Not sure if this works for Wisps, I only tested it on Shankles. This might also work for sleep.)
EDIT: Shankles seem to be immune to sleep. I need to test this further to confirm the immunity, though. Still not sure about Wisps.