Remain on level one and get enough experience points to buy the Magnum. Kill all the zombies on that level. Do not advance through the levels too fast. Go very slow and kill all of the zombies. Eventually you will have enough experience points to buy everything. When you start, kill twenty zombies to gain 200 experience points. Buy the Magnum in the first round. Keep killing zombies until you reach 2,000 experience and buy Turret Gun, also in the first round. Place it next to your "unlimited" Turret Guns and choose the "Kill Some Zombies" mission. Every kill the Turret Gun gets adds to your experience.