The tombstones can block the player's Wall-nut and the Desert Death Zombie can summon quicksand to boost zombies, increasing the difficulty. Plus, the level only gives the player Wall-nuts, which can be tough for a horde of zombies at the final wave. Plus, the Buckethead Zombie will give the player trouble getting stars at the final wave.
Try to destroy the tombstones first. Then destroy the zombies. Aim for the zombies that are the closest to the flowers, the toughest zombies and especially Desert Death Zombie.