Mirror-Shroom is a Bounty Hunt character in Plants vs. Zombies: Battle for Neighborville. They come in groups of three and resemble Night Caps wearing the "Crystal Crusader" Set. They can be fought in the Town Center, with a one-minute time limit.
Defeating them unlocks the Mirror, Mirror on the Shrooms medal.
This is one of the Bounty Hunts that is recommended to be played by two or more people, although completing this in solo is still possible. The one-minute time limit combined with the fact that these mushrooms can remain invisible for extended periods of time is the primary reason why it's very difficult to complete in a single player. To do that, use the Space Cadet and transform it into Space Station before starting the bounty hunt. After that, go to the left side of the overhang and wait until they become visible, then immediately use the Crater Maker to quickly vanquish them before they turn invisible again. If you run out of time on the Crater Maker, use Space Force to hasten the cooldown and take out the rest of the Mirror-Shrooms before the timer runs out. This can take several attempts to complete, especially when they keep distancing themselves from each other, which makes it harder to pursue them before they vanish. Although they do leave some trails and are vulnerable while they're invisible, these trails are not that noticeable, especially with the dim lighting.
Alternatively, you could try playing as the Engineer. His splash damage allows him to easily hit the Mirror-Shrooms, even when they are invisible, and can potentially hit multiple at once. Combined with Heavy Helper, the Engineer's main attack can easily whittle down their health. Bullhorn Swarm can be used to prevent the mushrooms from moving so much, while Double Time can allow you to quickly escape to regenerate your health if it gets low.
- Oddly enough, Bart Barian's tip implies there's only two of them, rather than three.