The Lord of Lies Belial is
He is actually one of the hardest boss'es in in my opinion, he has 3 phases.
1st phase:Belial will summon some Veiled Sentinels to attack you, they are quite easy.
2nd phase: Belial will jump down to fight you using abilities such as Fly Swarms (average damage), Melee attack (low physical damage) and Reinforcements which is calling snakemen to aid, they drop health globes.
3rd phaseHere is where things get rough
Fist slam (can be seen comming by a green ring)
Meteor strike (EXTREME DAMAGE AVOID!) to see them comming green rings will appear all over
Breath- will breath bad breath from one side of the arena to the other dealing damage
To read more on Belial and his ablities check the Diablo Wiki:
http://diablo.wikia.com/wiki/BelialEveryone 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.