On this page you can find various skill/class builds. All Builds are tested to be viable in Nightmare difficulty.
Here’s my current Archer build on Nightmare Difficulty:
Archery Tree: Death from Above / Long Shot / First Blood / Explosive Shot / Chain Reaction / Pincushion / Full Draw / Stunning Shot
Sabotage Tree: Poisoned Weapons / Infected Wounds
Subterfuge Tree: Stealth / Easy to Miss