How do you beat the robots on zombie farm?

Make sure your Zombies are at the highest level (combinations, with full mutations). Then send them in the same way you sent them in to the Ninjas. They are not any more difficult.

It is CRUCIAL to rapidly (very rapidly, frantically) tap the eyes of the big Old McDonnell robot in the background till he goes away temporarily. Otherwise, he will one-shot kill your zombies with his laser eyes. As the last robot comes down, the big Old McDonnell robot begins moving, making it even harder.

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