Your first opportunity to unlock this achievement will come on level 2-8.
Before you start the level, make sure you select the hypnoshroom. It will cost you 75 suns to plant.
The lead dancer zombie will appear twice during the level. The first appearance will occur relatively early, while the second will occur during the final wave. I recommend going for the achievement during his first appearance.
When the lead dancer zombie appears, he will be surrounded by four other dancing zombies. The lead dancer will be in the middle wearing a 1970's disco outfit.
The achievement requires you to hypnotize the lead dancer with the hypnoshroom. In order to hypnotize him, he must eat the hypnoshroom. Therefore, plant the mushroom directly in his path but slightly behind the zombie that is in front of the lead dancer zombie.
The achievement will unlock as soon as he is done consuming the shroom. In addition, the lead dancer zombie will turnaround and head in the wrong direction.