Organize your team by inspecting every character that you can include. Tap each of them to check their stats, and especially their effective range. Place characters with long-ranged sniper rifles far away from zombies. Characters that use axes or other melee weapons should be placed closer to zombies. Never attempt to tank attacking zombies. Always find a space to run to and then counter attack later.
Upgrades
Enhance your primary character by sacrificing lower priority characters to improve their overall stats. Focus on enhancing characters that are of the rarest tier as well as having the strongest statistics upon improvement. After reaching the maximum level for enhancing, you can now use evolution to further upgrade your character. Through evolution, the level of your character will be reset to level 1 and their overall stats will barely increase. Their biggest improvement will appear during the first enhancement after they evolve. After that, the character will level up and gain stats rapidly.
Zombie juices
Grind previously completed levels to stock up on zombie juices. This resource is required for your enhancements.