Are you a single Borderlands 2 player? Do you avoid online co-op like the plague and just wander Pandora by yourself? Well then you are missing out quite a large part of the game including raid bosses.
Raid bosses are incredibly strong foes that you have zero chance of taking down by yourself and who are even hard when there are 4 of you ganging up on them.
The latest boss, Dexidous the Invincible, to be added to Borderlands 2 came courtesy of Sir Hammerlocks Big Game Hunt DLC and is structured in such a way that you cannot even find the boss unless you have 4 high ranked players in the game.
Check out this IGN video that shows you how to find the boss and then how to get killed by the boss.