Permanent Shadow Form

Permanent Shadow Form (often abbreviated as perma-Shadow, perma SF) is a build that involves indefinitely maintaining an Assassin elite skill Shadow Form in order to remain invincible against spells and attacks. This build is often used to solo farm many difficult areas like the Underworld or the Fissure of Woe as well as farming different bosses in Hard Mode.

Viability
Since the introduction of PvE and PvP skill variations, the enchantment Shadow Form received a considerable buff with increased duration lasting over 30 seconds with reasonable investment of Shadow Arts attribute and an enchanting mod.

Usage
An Assassin with an Elementalist secondary can maintain Shadow Form by using Deadly Paradox and Glyph of Swiftness. Similarly, an Assassin or even a Mesmer can maintain it using Arcane Echo and Deadly Paradox although this version requires precise timing. The Assassin/Elementalist combination is often used due to a wide variety of Area of Effect spells as opposed to be Assassin/Mesmer and the Mesmer/Assassin variation. 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.

Warnings
Keep in mind that Shadow Form will not make the player 100% invincible, as Shadow Form does not protect the player from signets, touch skills (or any skill that is labeled Skill), traps, shouts, non-targeting Area of Effect skills, or skills that indirectly affect the player. For example, traps created by Obsidian Behemoths in the Underworld would mostly kill the player if he/she isn't careful. In addition, targeted area of effect spells can occasionally hit an unwary player - for example, if you kill an enemy minion master, the monsters may target his minions with AoE spells.

Discussions
This build has caused a lot of controversy within the community. Some players believe that Shadow Form allows farmers to farm in various difficult areas without any problems (particularly in Hard Mode and in the Underworld), which many consider this to be a gimmick build. Others believe that because of the nature of Shadow Form, the spell needs to be nerfed. Regardless, 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.