Combat
Stay behind the cover as much as possible, especially when facing many bandits. You can make a quick shot after leaving cover then getting back behind cover again. You can also change directions by looking to the other side as quickly as possible before getting back behind cover.
Easy rewards
Grind previously completed levels and memorize where the enemies appear. After repeatedly playing the level, increase its difficulty setting to get better rewards. The enemies will still appear from the same locations, regardless of difficulty.
Upgrades
Spend coins on new primary and second weapons and upgrade that gun that you are currently using. A good starting purchase is Grandpa's Gun, as it is relatively cheap and has more firepower when compared to other weapons such as the Rubbish Gun. Also upgrade your secondary gun to add more firepower, accuracy, and ammo clip.