The Zombie Chickens may pose a problem quickly. If the player does not take them out fast, they will quickly eat the defenses the player has set up.
The Pianist Zombies in this level pose multiple threats. Not only do they have the ability to make Cowboy Zombies, Conehead Cowboy Zombies and Buckethead Cowboy Zombies switch lanes using their music, they also are able to flatten the player's plants. Because of their dense health, they are one of the main threats in this level.
The Conehead Cowboys are Level 2, which means one of them is even tougher than a Buckethead Cowboy. They'll be a big threat if the player haven't upgraded their plants to Level 2 yet.
Lightning Reed should be chosen to deal with the huge number of Zombie Chickens and the Spikeweed and/or the Spikerock should be chosen to take care of the Pianist Zombies.