This build was originally intended as a monk-hunting build for TA, but I now use it in PvE all the time too. It makes short work of most elementalists (it's especially fun to use in The Crystal Desert).

Skills
Mark of Instability (No Attribute)
Beguiling Haze (Shadow Arts: Elite)
Black Lotus Strike (Critical Strikes)
Twisting Fangs (Critical Strikes)
Falling Spider (Dagger Mastery)
Critical Agility (Sunspear Title Track: PvE only) OR
Critical Defences (Critical Strikes)
Shadow Refuge (Shadow Arts)
Resurrection skill

Attributes
Dagger Mastery: 8 + 1
Shadow Arts: 12 + 1 + 1
Critical Strikes: 10 + 1

Equipment
Radiant Insignia x 5
Rune of Minor Shadow Arts
Shadowy Mask
Rune of Minor Critical Strikes
Rune of Minor Dagger Mastery
Rune of Attunement
Rune of Major Vigor

Daggers with + 5 energy.
I use Cruel Sai of Enchanting, but for this build you may find a Dagger Handle of Fortitude or Defense serves well. Equally you may prefer a Zealous or Sundering Dagger Tang.

Use
Choose your target (a monk or a fire ele to begin with) then attack starting at 1 and ending at 5.
This will:
Interrupt and Daze your target (Beguiling Haze)
Cause Bleeding, Deep Wound and Knock Down
Poison.

Keep Shadow of Refuge up when you can and as soon as you can cast Critical Agility or Critical Defences. You will pull off enough critical hits that these skills will keep refreshing (most of the time) until combat is over.

Counters
If your hex is removed you are useless as your dagger sequence needs a hex. It is a good insurance policy to make sure someone else in your party has a hex.
Spell Breaker is particularly annoying against this build.
Obviously blind is problematic.
You are VERY unlikely to come up against this, but Wail of Doom is not a lot of fun.

The recent skill balance has changed Black Lotus Strike from an offhand to a lead attack. This build will no longer work as intended.