Enter the "Multiplayer" menu. Select "Unlockables", then enter one of the following codes.
Reach the indicated rank to unlock the corresponding weapon.
Complete the indicated task in Multi-player mode to unlock the corresponding trophy.
Complete the indicated task in Multi-player mode to unlock the corresponding wildcard.
Complete each condition to get the allotted Trophies.
Reach Rank 25 in online gameplay to unlock an exclusive gamer picture.
Aside from repairing vehicles, you can use the power tool to break down walls/obstacles, kill enemies and even disassemble enemy vehicles.
Quick switching will save you a lot of time especially when dealing with multiple enemies. For the auto-injector, once the screen tilts and you hear the hiss, you can switch to another item or weapon. For weapons, once the magazine total is full, you can select another weapon or item and your gun will be ready next time you select it.
Instead of using grenades to destroy walls or fences, you can use your knife instead.