Permanent Shadow Form

Permanent Shadow Form (often abbreviated as perma-Shadow or perma SF) refers to any build that focuses on indefinitely maintaining Shadow Form in order to remain invincible against spells and attacks. Such builds are often used to solo farm difficult areas like the Underworld or the Fissure of Woe and to solo boss farm in Hard Mode.

Viability
As a result of the May 22nd, 2008 update, Shadow Form received a considerable buff. Its duration was increased and it can now lasts over 30 seconds with a reasonable investment in Shadow Arts and an enchanting mod.

Usage
Shadow Form can be maintained indefinitely by combining Deadly Paradox with either Glyph of Swiftness or Arcane Echo (although the latter version requires precise timing). The Assassin/Elementalist combination is particularly powerful (as compared to the Assassin/Mesmer combination) because it has access to a wide variety of powerful Area of Effect spells. Because maintaining Shadow Form is very energy intensive, it is not uncommon for builds to use Channeling, Glyph of Lesser Energy, and/or Spirit of Failure to maintain a high energy pool.

Counters
Keep in mind that Shadow Form will not make a player 100% invincible, as Shadow Form does not protect him or her from non-Spell skills, non-targeting Area of Effect, or skills that indirectly affect the player. For example, traps created by Obsidian Behemoths in the Underworld will mostly kill a Permanent Shadow Form Farmer if he or she isn't careful.

Controversy
The possibility of indefinitely maintaining Shadow Form has caused a lot of controversy within the community. Some players believe that Shadow Form allows players to farm in various difficult areas without effort (particularly in Hard Mode and in the Underworld) and many players consider such builds to be a gimmick. ArenaNet has officially responded to this issue in their June 18th Developer's update stating that they will "evaluate the high efficiency of Shadow Form farming builds" and will "monitor this issue and may make adjustments in the near future." As such, the fate of this build may appear to be very grim, since ArenaNet had made hostile changes towards farming in their previous updates. The July 2nd update added a 50% damage penalty to Shadow Form, thus making permanent shadow form farming much less effective, and time consuming.