Do what you need first. Kill the first zombies with cheap attack plants. Don't forget to use Sod to plant on pathways.
Archer Zombies appear a lot in this level, that means it's a perfect time for Carrot Missile. Plant Peashooters, Carrot Missiles and an another attack plant from your friend, to build your attack. Upgrade the Sunflowers into Twin Sunflowers and the Peashooters into Repeaters if you can. Plant Wall-nuts to protect you from Archer Zombies and Shield Guardian Zombies.