It'd be kind of like a ditto from pokemon, but more like the bogart from Harry Potter. The classic shape shifting gel.
There would be one universal set of monsters that all shapeshifters would become; this monster would be on the same level as the shapeshifter for instance, mecha knights, grievers and gremlin thwackers are all elite monsters, thus would be fair game for a fully healed shapeshifter. However, as they get damaged, shapeshifters would only be able to select lesser monsters. So the mecha knight you were fighting might turn into a scuttlebot after a few hits. All would deal split normal/piercing or pure piercing, always be weak to shadow, and would retain the coloration of a shapeshifter.
The shapeshifter would fall back and revert to it's true form (but quickly re-transform) when either damaged enough to be set back a level, or knocked down and canceled.
Status variants of the shapeshifter would obviously become status variants of other monsters.
I think these should be able to only select monsters with meelee attacks, or at least only perform meelee attack. I think this could serve as a miniboss like Tortodrone, Jelly Green Giant, or a Battlepod.
Now I think of it, Jelly/Jolly Green Giant is most probably the slime Elite.