This will come with normal progression of the game. There are 50 total waves, or levels, and making it through the first 25 is fairly easy. The best method I can recommend is upgrading the farmer allies by spending coins on them in the shop between levels. Focus on upgrading them, upgrading your powers, and upgrading your bow/sword the most. A general strategy is to spam the farmer allies at every 5 upgrade power you get, as they will act as distractions for the advancing zombies. The two powers I focused on were Katana Slash and Lethargy (this power slows the enemies for a short time). While the zombies focus on the farmers, spam your powers and stay within shooting distance of your bow, and the zombies shouldn't prove too tough.