Plant Sun-shrooms and Puff-shrooms for an easy defense at first.
Use the same strategy in Level 4-8, but use the Cattail to pop Balloon Zombies (if you bought it) instead of Cactus. Cattails are more efficient in Pool or Fog levels. Try to plant at least four Cattails in this level for it to be really easy.
Use Planterns or Blovers (if you do not have Cattail, as stated before) if you want to see through the fog. Just remember that seeing through the fog is a luxury, as it is not required.