I advise starting the game in co-op. Completion of the game in co-op will also net you an achievement and help against the higher difficulties.
Look at each weapon at the D.O.I. before you start a mission as each weapon has a different trait with different characters.
Once you unlock the babies I advise you always carry one. They can be very helpful.
TIP: You need to collect Case Files to unlock the last projectile. However if you have a friend with all of the items unlocked he can drop one in a mission by picking up a new weapon and then you may take his old weapon. Once it's in your hand it is yours and you may quit back to the level select and go for a new weapon.
Case 1: Baseball, Paint Can, Hammer and Snow Globe
Case 2: Boomerang and Propane Tank
Case 3: Dumbbell, Golf Ball and Model Rocket
Case 6: Bowling Ball, Knife, Kerosene Lamp and Chicken
Case 7: Letter Opener, Demon Teeth, Wrench and Spell Book
Case 9: Mating Bone, Desk Lamp, Harpoon and Hair Spray
Case 10: Salt Shaker, Fire Extinguisher, Gas Can and Magic 8-Ball
Liquor Bottle - 5th Case File
Zombie Arms - 6th Case File
Demon Head - 7th Case File
Wedding Cake - 8th Case File
Minus Boss Levels, Every case has a hidden file (it's random if they are on the stage)
Break everything in search for it and keep playing the same level over and over until you find it. It's a floating glowing file. You'll know it when you see it.
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