Ok, this may sound silly, but there is actually a proper way to grabbing drops that fall from bosses. A lot of people complain that they don't get any drops from bosses, well here is how you do it!
There is no guide to know how what gear will fall where, This is more to get you to be able to pickup whatever drops. When you are attacking a boss, you will notice a large orange circle on the ground. That would be the area that the boss occupies and this is what you base your positioning on. When