This is my go to build, and is what I use in most Nightmares. There are a number of slight variations I use for specific situations and I'll try to cover all of those as well.
Before we jump in, I suggest binding your Action Button 6 & 7 to Q and E. Timing is everything and keeping your keyboard hand in a tight little group is always helpful.
- Illusion (Tier 1 Chaos, Chance Tree)
- Brandish (Tier 2 Blades, Running of the Jagged Tree)
- Martial Discipline (Tier 1 Blades, Technique Tree)
- Stoicism (Turbulence Misc Tree)
- Smoke and Mirrors (Tier 2 Chaos, Shell Game Tree)
- (Q key). Reality Fracture ( Tier 2 Chaos, The Fourth Wall)
- (E key). Escalation ( Tier 1 Chaos, Chance Tree)
- Agitator (Turbulence Misc Tree)
- Intensity (Tier 1 Chaos, Chance Tree)
- Brandishing Support (Tier 2 Blades, Running of the Jagged Tree)
- Hardcase (Tier 2 Shotguns, Hunkering Down Tree)
- Inevitability (Tier 2 Chaos, Building Blocks Tree)
- Master of Illusions (Tier 1 Chaos, Chance Tree)
- Sixth Sense (Tier 2 Blades, Sharpening the Senses Tree)
The Elite Passive is debatable. You could go for a situational passive like Forged in Fire, or Prodigal.
If I need a taunt (for example on fights where I might be Impaired, such as in Polaris) I'll swap out Martial Discipline for Provoke.
If I need more AoE Hate, which is really any fight that has Adds or Trash, I'll swap out Brandish for Crimson Theater and Brandishing Support for either Shadow Play or Fuel to the Fire. (Fuel to the fire if I still need more Hate, Shadow Play if I don't need more Hate.
If I need an interrupt I'll swap Martial Discipline or Smoke and Mirrors for Trial by Swords.
If Hate isn't as much of an issues, especially if AoE Hate isn't an issue, but survivability is, I'll swap Reality Fracture for Karma.
I always open with Stoicism running. This gives me a head start on Hate, and helps me hold aggro while I position the boss. Your first attack should be 2x Escalation. Even though you begin combat with 5 Blades resource, you want to apply Weakened, then hit the Weakened target. This allows Intensity, Inevitability and Hardcase to begin working.
Next move on to Brandish or Crimson Theater (depending on which you are using).
From here just spam Escalation and use Reality Fracture when you have 5 Chaos Resources and Crimson Theater/Brandish when you have 5 stacks of Blades Resources.
I use Stoicism whenever it is off cooldown, as soon as it is off cooldown (Unless of course its a phase shift, or the boss is doing something other then hitting me). I also tend to keep Illusion on cooldown, but I save Smoke and Mirrors and Martial Discipline for when I'm in trouble.
Your primary stat is Evade and Defense Rating (Yggdrasil Runes for Evade, Wedjat Runes for Defense). Evade is obviously great. Something completely missing you is really nice. However, relying completely on Evades means your damage intake will be very spiky. You'll find yourself dying because the healer looked away for 3 seconds and you failed to evade 3 times in a row. Defense counters this by preventing bosses from Critting, this makes the damage you do take more steady and predictable.