For this you have to complete the game on hard to obtain the gold medals. You can replay each level and obtain your missing medals for "Destroyed objects" "searched things" "Zombie Kills" and "Numberof Death."
Go for Objects and Things first, if you manage it not to die its good, if not, dont worry you can review your teammates if they die. In my opinion Mike and Foxy are the best characters to play with. With mike build the poo trap or the box trap to make the zombies kill each other and with foxy of course the bomb trap and her weapon if available.
Also let the Bots do the work if your not alone and focus on buildung trap.