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Do the lander glitch by the PHD Flopper. When you get on the roof have your friend call the lander by the Speed Cola. After you see the lander gate start to go up jump on it and you will now be on its barrier try to stay on it until you get to the power building and jump on the power building. ENJOY
EDIT:Be careful on the power building the zombies somehow seem to spawn up there not alotbut it does happen.