-Stay on level 1 and get enough experience points to purchase the Magnum. Kill all the zombies on that level but 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 starting, kill 20 zombies to gain 200 experience points. Purchase 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 select the "Kill Some Zombies" mission. Every kill the Turret Gun gets adds to your experience.