How do you pass night level 8 in Plants vs Zombies?

Option 1: Defense.

Dancer zombies will stop moving forward when either they, or their "support dancers" hit something (i.e. a plant). So one option might be to place a Wall-nut to allow for 3 rows of space. This should be enough space to allow the Dancer Zombie to come in, spawn a Support Dancer, and still be in front of the line of Wall-nuts.

Option 2: Assimilation.

Another option is to use the Hypno-shroom to turn the Dancer Zombie into a friendly unit. When a dancer zombie spawns, you can place a Hypno-shroom down and the Dancer Zombie will eat it (they can even eat it before they spawn their Support Dancers), at which point they will turn and start attacking other zombies. Dancer zombies have a faster attack speed than other zombies, so they can survive a little longer, and even spawn Support Dancers to help as well.

Option 3: Both.

While it does limit your selection, you can go with both. This video only uses the Hypno-Shroom, but they use both the SunShroom and the Sunflower or the DoomShroom, and I personally have been able to survive with just the SunShroom, and I find the DoomShroom to be (while effective) less effective than the Wall-nut.

What I sometimes do is if there is more than 1 dancer zombie onscreen, I use a ice shroom to freeze all of them. The idea here is so they won't spawn more backups so you can concentrate on killing the backup dancers and then the lead dancers.