The Tomb Raiser Zombies are tough, and Ra Zombies can deal a lot of damage.
Upgrade your plants' level by playing previous levels. Then upgrade them more with Puzzle Pieces and the Oven in your house.
Drag as many plants as you can into the lawn. Use other users' plants to have even more power. Note that you should place the plants with the strongest attack in the bottom lane, and a defensive plant in the upper lane, in order to protect your plants.