Punching a mimic with a worn/wielded ring of protection from shape changers makes them die instantly.

Assuming an implementation that lets monsters wear rings and lets you pick and choose what your pet equips: You can have your pets wear an amulet of unchanging to make them immune from being polymorphed or shapeshifting; possibly the rings of sustain ability could protect from both as well, and having them wear protection from shape changers could be used to prevent them from shapeshifting, if they are a shapeshifter.

Putting on a ring of protection from shape changers and forcing a shapechanged monster somewhere on the level to revert to its normal form wakes it up if it’s asleep.