Belial is the guy at the end of act 2, and the last part is really hard. I never seem to get his life very low. How do you beat him as a monk?
Kill all the worms which is trivial.
Focus on Belial and let your companions handle the little guys for the most part. Make sure at the end of this round you are full on spirit or else you'll be in much worse shape.
From what I can tell, Belial has 3 main abilities:
It requires a little concentration, but if you counter each one individually, it's much easier.
When he whacks you, you are in his melee range and it doesn't hit very hard, so you can basically ignore it.
When he spews green stuff, make sure you 300 life and you should live through it.
The green exploding void zones are the hard part. These will tag you for bleeding damage while you are standing in them and then they pop you for 500-600 when they explode. The only real strategy for these is to run away from them. I have found that running around the outer edge of his zone gives you the most room to work with and tends to be a lot less busy so you can see yourself the whole time.
This is not a DPS fight! Focus on keeping yourself at full life. I used the aura that heals you when you use spirit and the one that gives you 100 extra spirit. I don't think the mantras I had at this point are particularly useful for this fight, but I used the one that increases dodge to help with the first 2 stages. I also used the aura companion spell and the heal party spell. I had the enchantress follower with me.
The basic strategy is to keep your spirit capped off, and as soon as you take damage, blow some spirit. During the slow parts, run up to him and build your spirit back up. Don't forget to use your potion if you get into trouble and don't worry about your companions, they will figure it out. The last piece of advice is to not go crazy and blow all your heals when you take damage. If he hits you with a whack as you are lighting up your spirit orb or you take a green spit, just cast a single damage spell that uses spirit and save the big cool downs for when you are running away from the green voids.
Lastly, health globes do pop up in melee range quite often. It's more important to keep yourself topped off than it is to DPS.
Nick's answer about how the fight works is awesome, but to add to it here are some of the specific spells I found useful during the fight
The important bit was to save your spirit for heals, and just auto-attack for damage and spirit generation.
And a side note, be sure your best quality health potion is equipped in your Q slot. I didn't notice at first that it doesn't automatically use the best health potion when you hit Q, and you have to actually change which health potion to use by dragging the one you want onto your skill bar.
Inferno mode is a bit problematic, since you have 4 minutes in phase 3.
With adequate gear (800+ loh, 900+ all res, 20k+hp and 5k+ armor, at least 14-ish k unbuffed dps), and with this setup you should have little problems.
Phase 1&2: Just stand in a corner and spam thousand fists, using other skills as needed. The huge knockback of the 3rd hit will migitate a lot of dmg. With above stated stats u shouldn't have to move from anything belial/trash throws at you in phase 1 & 2.
Phase3: You should be able to survive a hit or two from belials slam and be healed to full by the loh in almost no time. If you see you wont use breath of life, or a potion. If none is available, don't use serenity, just move a bit to the side and continue dpsing. During the voidzone part, stop dpsing and move out of the pools. If you get cornered in pop serenity and move to a safe spot. After, just continue to dps following all stated above.
Also, during the 3rd phase, it is imperative you keep the sweeping wind up at all times, use blinding flash whenever it's up and pop the mantra whenever you can. And keep the dmg buff from breath up, although it last preety long and probability is you wont even have to look at it.
After dying a couple of times in this fight, I finally went with the passives Exhalted Soul and Transcendence. Keeping a large Sprit pool available (and a couple of health potions) was the key for me to keep alive in this fight! DPS and keep out of the bad stuff.
I soloed him in a party of 3 because the other 2 died. Use serenity with the rune that makes it heal you and you win