The Buckethead Mummy and Tomb Raiser Zombies are tough, plus, Flag Mummy Zombie can heal them. Strong attack plants are definitely needed for this level.
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 players' plants to have even more power. Note that you should place the plant with the strongest attack in the middle lane, to destroy the Buckethead Mummy and Flag Mummy Zombie quicker.