From my understanding.... which isn't a lot on bosses yet lol. Coop bosses are much stronger and have more health. They are unlocked by beating certain levels of the Single bosses. From a post by one of the Admin, Single bosses are meant to be fought alone even though you can ask for help, Coop bosses aren't. Although I guess the name makes that sorta obvious. Copy and pasted from the Help in game. Single Bosses Each location will have its own Single Boss. Once a fight is started with a Single Boss, you will have 2 days to defeat it. You are only allowed to have 1 Single Boss fight at a time but can be active in an unlimited number of your friend's Single Bosses. Each Single Boss a number of achievements associated with killing specific levels. Coop Bosses Coop bosses are tougher to kill. Each boss will have its own limit on how many friends can help. Once a battle is started with a Coop Boss you will have a battle time limit of 4 days. You are only allowed to have 1 Coop Boss battle at a time but can help your friends with their coop bosses.
Calder left very good basic info. Additional info: Single Bosses must be beaten one time to move on to the next region. Coop Bosses are completely optional, but drop good loot, skill points (levels 1, 5, and 10), and sometimes UN points when beaten.
