Plant Coconut Cannons immediately when you have enough sun. Five Coconut Cannons are needed (You might want to use Plant Food to Sunflower or Wall-nut to do this). It's nice to fire Coconut Cannons when there are a lot of zombies.
The great combo (Swashbuckler Zombie + Seagull Zombie + Imp Cannon + Pirate Captain Zombie) is now appear as your biggest threat, since they can pass your Wall-nuts easily (so you should use Plant Food to some of the Wall-nuts).
You can earn a lot of Plant Foods in this level so don't forget to use it.