The level was easy as the zombies were all weak. However, the player may have wanted to level up plants to minimize damage taken and maximize damage done.
Upgrade your plants' level by replaying previous levels. Then upgrade them more with Puzzle Pieces and the Oven in your house.
Strongest attack plants should be dragged onto the lawn. Ice plants as Snow Pea could be used to freeze enemies. Healing plants, like Twin Sunflower, or charge-attacking plants such as Iceberg Lettuce or Cherry Bomb, were optional.