Belial is the Act II final boss, and like every other boss in the game, you will most likely be fighting him 4 times. While you can essentially skip around, it's best to do them all so you can do the challenges while you're playing through the game to get Out of the Shadows.
Good Eye - Kill Belial without dying and without getting hit by his projectile or meteor attacks
Fist Missed - Kill Belial without getting hit by his fist attack
Why'd it Have to Be Snakes? - Kill 10 Veiled Sentinels in the first 30 seconds of starting the fight with Belial on Nightmare difficulty or higher
Bad Breath - Kill Belial without getting hit by his breath attack on Inferno difficulty