This trophy says open 50 Chests but infact you don't have to open just chests this will also count for all types of things you loot from such as barrels, sacks and much more.
Looting a dead body of an enemy you just killed does not count, but the random dead bodies around the world that you come across do count.
So really just loot everything you come across and you will get it before the end of act 1.