1. Don't use Glass Cannon!
I know you are very used to using the Barbarian Weapon Mastery which gives you a straight 15% damage bonus. But Glass Cannon also comes with a penalty of -10% to armour and resists. With prismatic armour nerfed to sheds and without that natural 30% damage reduction, you can't afford to lose those defences!
2. Do your research!
Diablo3 is simple to play, but very hard to master, just like Chess! No, even harder to master than Chess because Diablo3 has wonderful hidden mechanics which is not found in any game descriptions, like Skill Proc Rates, DOT tick rates for different attack speed and so on. Truly a wonderfully complex mastery piece!
3. Don't use more than one armour spell!
Don't put more than one armour spell, Prismatic, Ice or Storm in your hotkeybar! The reason is because casting one will cancel any other already in effect. Just treat wizards as having two less skills.
4. Don't use more than one channel spell!
Don't put more than one channel spell, Ray of Frost, Disintegrate or Arcane Torrent in your hotkeybar! I know you like Dragonball. Me too. But just like Goku never charges two kamehameha at the same time, you can't use two channel spells at the same time. Just treat wizards as having two less skills.
5. In fact, don't use any channel spell!
I know we all want to be super heros like Batman, Goku or Optimus Prime. But Witch Doctors have sort of gotten the bat thing covered and barbarian's have got the transforming-and-unstoppably-rolling-things-over-at-100 mph thing down. So we are left with Goku. However there is a problem. Unlike most of Goku's opponents who are perfectly willing to wait 3 or 4 episodes for Goku to charge up his kamehameha, the enemies in Diablo 3 do no such thing. If you stand still for more than 2 seconds, you will find them invading your personal space!
I will update this guide as and when i have more tips. Hope you beat Nightmare soon.