The Explorer Zombie is extremely tough and deal high damage, along with the tough Tomb Raiser Zombies, it can cause great havoc to the player's defenses.
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 middle lane, in order to kill Explorer Zombie.