Most Thieves excel at single target burst to get the best out of their builds. One weapon, the shortbow, does have the proper AoE capability to make this class decent at tagging large groups of enemies. This build focuses on the ability for Thieves to spam skills through initiative and capitalizing on shortbow's ability to hit multiple enemies in one go.
The main goal of this trait set-up is to give you the ability to spam attacks with your shortbow, and to stay alive. Roughly forty trait points are spent to increase your initiative gain and damage dealt. Twenty points are spent of capitalizing on the amount of dodging you will do by spreading Caltrops and giving you might. Last ten points are spent on a splash of condition cleansing which is necessary for thieves.
Your utilities are mainly for survivability and a small bit of offensive power. Smoke Screen gives you a much needed reprieve from ranged attacks while also providing a blind if enemies get too close. Signet of Shadows gives the build some sorely needed speed outside of combat. Infiltrator's Signet is the only stun-breaker and provides more Initiative gain for more damage output. Finally Dagger Storm is absolutely one of the best skills to use in a large group of mobs to get the most damage out and tag plenty of enemies for kill credit.
The name of this game is use the second shortbow skill as often as possible over groups of enemies. Detonating your Cluster Bomb over a large group hits as many enemies as possible and deals a decent amount of damage to give you kill credit. Letting the Cluster Bomb just hit a smaller group of enemies will undeniably give you credit, but should only really be used in a tighter group of enemies.
Whenever possible make sure you steal from an enemy and use the skill you gain quickly. This will give you fury which will further accent your damage output for kill credit. In addition to this every time you steal you get back some initiative so you can continue to spam out your shortbow skills.