A few of the early bosses, Just the singles until Russia have a weakness to particular items, that usually include the boss drops from the single boss before. This weakness ony does you any good if you dont have a stronger item that what the weakness increases your item to. So for example, if you can equip all UN items, or boss drops from higher locations, The weakness does you no good.
The weakness is shown in a tab in the area next to the area that shows the damage dealt by you and your squad. Here is an image for reference.
Last edited: Jan 31, 2013