Explorer Zombie dealt much damage to the player's plants, as they're all protected by four Mummy Zombies so they couldnt be killed quickly. Using a defensive plant was a good choice.
Upgrade your plants' level by playing previous levels. Then upgrade them more with Puzzle Pieces and the Oven in your house. Hunt more Puzzle Pieces from previous levels.
Strongest attack plants should be dragged onto the lawn. Ice plants as Snow Pea could be used to freeze enemies. Use defensive plants, such as Wall-nut, to protect other plants. You could also use other players' plants for stronger attack power and defense.