Again, start out by planting Sun-shrooms and Puff-shrooms for basic defense and sun gathering at the beginning of the level. Plant at least three columns of Puff-shrooms and two columns of Sun-shrooms if possible.
If you are overwhelmed early on, use the Ice-shroom. It will stall the zombies for long enough to be defeated.
When your defenses become more fully developed, plant a few Repeaters for a stronger defense.