Farming armor is not as hard as it sounds, the only things required are a zombie spawner, water, a hopper, and a chest. First, surround the dungeon with the zombie spawner completely with walls. Then, place water on one side so it creates a stream that runs to the other side. Where the water stops flowing, dig a 4 block deep hole in the ground. Place water so it fills the bottom two blocks of the hole. Dig two more blocks down and place a chest at the bottom and then a hopper above it. Finally, dig out a room with that chest in it so you can access the chest much easier. When a zombie spawns with armor or something in its hand, it will walk towards you while you are in that room. If you placed the water stream in the right place, it will fall into the stream and be pushed down into the hole. It will stay at the bottom and after a few moments, it will turn into a drowned. It then instantly drops all of the items that it is attached to and the armor will have full durability. It should drop into the hopper and be moved into the chest, where you can grab it. You may also want to make a way that you can collect the armor in person because the armor sometimes floats up to the top of the water and avoids getting sucked up by the hopper.