Duplicate Melee Weapons
To duplicate a melee weapon, start by equipping it from your inventory. Throw the weapon and while it is still in the air, quickly press the appropriate button to access your inventory. Unequip the weapon that you just tossed, then exit out of your inventory. Now you can pick up the weapon and your inventory will show that you now have two of that weapon. This is a great way to earn money quickly, by selling duplicated equipment.
Infinite Character Inventory
Fill your weapon inventory but leave one empty slot. Initiate a Trade with a friend and have them give you two Weapons. Now you can pick up, receive, and hold as many weapons as you want. Be sure to always have at least one more than the maximum number of weapons in your Character Inventory at all times.
Canned Food Exploit
In an early chapter, you'll meet a girl who asks you to scrounge along the beach for water and food. She'll next ask for canned food. Once you retrieve around 10 or 20 cans, toss it on the ground and then kill yourself. As the recover process follows, quickly load your last checkpoint and access your inventory. Scroll down and you should find that the cans are back in your inventory. They also should be on the ground where you left them before loading your checkpoint save. You can keep repeating the process to mass a huge pile of cans that you can use to gain experience and money.
Carry Additional Weapons
When you are playing as John Morgan, enter rage mode. John will pull out a staff, which is separate from his standard inventory. If your inventory is full at that point and another weapon pickup is nearby, you can quickly select that other weapon to replace the baton that John is carrying. That will add an additional weapon to your inventory, so that you are now carrying more than the usual maximum of 18 weapons. You can continue to expand your inventory in this manner, by activating rage mode and swapping out the baton for addition items as your inventory swells.
Duplication Of Weapons
First you need a partner in coop throw the item that you want duplicated down.you will also need another machine gun/assult rifle pretty much anything when you hold the trigger down it fires bullets rapidly...next while shooting the weapon have your friend pick it up! It's harder to do diamonds because there soo small but if you have god guns duplicate them with friends!
Achiever 0 Finish all quests in the game
Mystery solved 0 Find all the secret files
Obtain level 50 0 Obtain level 50
Obtain level 70 0 Obtain level 70
Siege Hammer 0 Install a total of 5 barricades during hub defense
Dream Team 0 Complete all the main quests cooperatively with any number of partners
The Collector 0 Find 50% of collectibles
The Hoarder 0 Find all collectibles
Professional Tourist 0 Find all pages from the guide book
Twins 0 Kill the Twins
News junkie 0 Find all the volunteer's voice recordings
At Your Service 0 Rescue 20 NPCs that have been grabbed by monsters
The Hurt Mine Locker 0 Kill 10 monsters with one mine
BrainLess 0 Kill 50 monsters by smashing, decapitating or shooting them in the head
Hurler 0 Kill 100 monsters with thrown melee weapons
Bully 0 Kill 100 monsters with a kick
Jetboat 0 Kill 100 monsters using the boat's speed boost
Juggernaut 0 Kill 100 enemies using charge attacks
Better than the arm 0 Kill 25 monsters with a chainsaw
It's an Order 0 Mark 100 opponents which were then killed by your co-op partners
King among Kings 0 Kill special enemies: Grenadier, Thug, Wrestler, Infected, Suicider, Screamer, Butcher, Floater
Hunter-Seeker 0 Find and kill all named champion monsters located in dead zones