Belail , the Lord of Lies is the last boss of Act 2 in Diablo 3. He can seem hard to beat at first, but once you know the tactic he will hardly do any damage to you and he can be farmed quickly. Once you get the hang of it you can get to him and kill him in about 7 minutes, making him a very desirable boss to farm. If you feel like getting some gear upgrades / levels before going on to act 1, farming Belial is a good idea. The whole quests awards you with about 12000 XP on normal.
1: Select the last quest of Act 2 “Lord of Lies” and enter the game.