It is the hardest boss out of all the seasons.
After half-way of his health down, he grabs Penny (From PvZ2) And summons zombies from other times (From PvZ2).
A quarter more damage done and he transforms into PvZ2 Zombots, with the zombies from that world/time zone. He turns into Sphinx-Inator, then Zombot Plank Walker, then, Zombot War Wagon, then Zombot Tomorrow-tron, then Zombot Dark Dragon, then Zombot Sharktronic Sub, then Zombot Tuskmaster 10,000 BC, then Zombot Aerostatic Gondola, then Zombot Multi-stage Masher, than finally, Zombot Dinotronic Mechasaur.
When you defeat all of them, Zomboss will be at the 3rd stage of his health, and the battle gets harder until you defeat him.
After you defeat him, he gets launched out of his seat as the Zombot explodes, and he goes for the players house. If the Player loses to him, you lose and have to restart the battle up to the 3rd stage of health. But if you defeat him (He's speedy but has the same health as a normal Zombie), he dies, and the credits roll.