Do not break open boxes or traffic cones unless you can use the health and ammo that they may contain. Your character will automatically use them, even if they are at full health or ammunition.
Combat
When attacked from both directions, circle around your enemies in order to face them all. Then, use the shotgun to take them out in groups. Focus on eliminating fast zombies as soon as possible, as they are likely to cause the most damage.
Upgrades
Enhancing your items and weapons to make them more effective. Focus on your top tier, multiple star, items because they will usually be in use for a longer period of time. Check a weapon's maximum level before leveling it up to avoid wasting resources on an item that is already close to its limit. Your inventory space is limited. Upgrade it often to have room for the weapons and equipment that are given as rewards when a level is completed. Make sure to keep your inventory in good shape by expending extra weapons and other items to upgrade your best equipment.
Weapons
When safe (for example, between enemy waves), use a melee weapon to save ammunition such to break objects or when only a few enemies are present. Use sniper rifles to eliminate many enemies at once. The amount of time required to set up a shot is worth the effort. Sniper rifles are also useful against Bosses. When possible, circle around a group of enemies and place your back against the left side of the screen to line up a sniper rifle shot easier. More enemies will often appear from the right side. Sack-carrying thief girls will occasionally appear between enemy waves. Equip a rapid-fire weapon, such as a pistol, to take them out quickly for large rewards.