The Lord of Sin Azmodan is
He can be a bit of a challenge, the longer the fight lasts, the more challenging he becomes.
Azmodan has abilites such as:
Melee attack
Bellyflop
Eye laser attack
Falling Corpses
Demon Gate
Pools of destruction
Globe of Annihilation
To read more on him or his abilities check the Diablo wiki:
http://diablo.wikia.com/wiki/Azmodan Everyone has their strategy to beat him, working in a group is the best way to go because as long as someone stays alive everyone can continue to respawn fighting him, if you are alone and die then you have to start from beginning.