When playing Grim difficulty this tip can help with the start of the level. At the start, shoot the 2 zombies in the barricade right at the start then proceed to blow the tank on the other roof. Dispense with the fat ones that survive the blast with powershots, then dash for that first crate. Crates drop random cash from 6,000-30,000. Keep starting new game to get 30,000. Clear the zombies ahead and opening up the chest by the alarmed car. You should have enough cash now to fully upgrade Power for your rifle.