How do you avoid Belial's poison attack?

The key is to just stop attacking and move the moment there's some green under your feet.

You can get back to the fight as soon as you're back in the clear, and you won't take any damage. What's more important, staying alive, or a bit more damage?

When he does his huge multiple poison attack, just don't stop moving until it's over. Keep moving and aim for the spots that either were just poisoned (so they're turning back to normal) or are currently normal. So long as she doesn't get hit, she can join in the fight again once it's over.

If she equips strafe, then she can keep attacking even while moving if she really feels like she needs to put out that extra DPS instead of staying alive.

Belial's poison attack only lasts a few seconds but it can be deadly. It took about three or four tries with me (Demon Hunter) and my buddy (Barbarian) on Nightmare.

I ended up using a Sentry that constantly attacked Belial even while we were avoiding the poison attack which seemed to help.

Definitely during the poison "phase" you should be more defensive and just try to avoid the green as much as possible, then focus your attacks when it is over.

Me and a friend are running the same combo and whenever I got close to dying or my friend felt that I was getting hit too often he popped mantra of healing with the Boon of Protection as active skill rune. It shields you from some of the damage while also giving you extended regen so its PERFECT for Belial!

Also, if you want to heal your friend up be sure to stand close to him while using breath of heaven (12 yard radius). I had tons of situations when i used vault to escape with low hp and barely got out of range and therefore couldnt get the healing.

If you guys feel you can't keep your hp up in the higher difficulties I would also give you the tip to use Serenity with the Peaceful Repose rune. Your friend should use Preparation with the Battle Scars rune. Both are basically a second hp pot for free every 20 second for you and every 45 for your demonhunter friend.