Some tips for the best experience during the Zombie Attack :

• Accumulate the maximum amount of supplies before Zombie Attack;
• Make sure all preppers and leaders are healed and not active before Zombie Attack starts;
• Split your supplies evenly between the four gates of your town. 
•  Add two preppers to each gate by double-clicking on the prepper. Make sure the preppers or leaders are your flamethrowers and sniper-based characters.
• Note that as you play through the event, you will gain more supplies that you can put into your gates and buildings. First priority is to keep supplies stocked int your gates. Supplies can diminish from gates when zombies damage them. 
• Occasionally during the event, there are zombies that get through your walls. To help counter the zombies, it its recommended to put a small number of supplies into your warehouse, diner, and either recycling center or chemists' trailer.
• Remember to put at least one prepper into these buildings as you put the supplies in. These buildings are secondary to your gates.