Bosses
Boss health scales with # of players at a rate of +80% base HP per each player past 1. For example, Evil's Protector has a base HP of 3000. With 3 players, its health total is 3000 + 2 * (3000 * 0.8) = 7800. Each boss is guaranteed to drop one of the items in the "drops one of the following" section. If you defeat the boss again, it will drop one of the remaining items until they have all been dropped. After that it will pick another random item and begin the cycle again.
There are 3 bosses must be defeated to beat the game. They are Pirate Captain to unlock Deep Caves, Sage and Grit to unlock Desert Temple and Fallen Wizard to unlock Incursions. Those bosses might just be enough to beat the game if you find a very rare structure in deep caves.