something like this?
====== Created Using Wizards of the Coast D&D Character Builder ======
level 1
Halfling, Rogue
Build: Brawny Rogue
Rogue Tactics: Cunning Sneak
Rogue: Sharpshooter Talent
Sharpshooter Talent: Sharpshooter Talent (Sling)
Background: Halfling - Banned (+2 to Stealth)
FINAL ABILITY SCORES
Str 13, Con 10, Dex 20, Int 8, Wis 10, Cha 15.
STARTING ABILITY SCORES
Str 13, Con 10, Dex 18, Int 8, Wis 10, Cha 13.
AC: 17 Fort: 11 Reflex: 17 Will: 12
HP: 22 Surges: 6 Surge Value: 5
TRAINED SKILLS
Stealth +12, Thievery +12, Streetwise +7, Dungeoneering +5, Perception +5, Acrobatics +12
UNTRAINED SKILLS
Arcana -1, Bluff +2, Diplomacy +2, Endurance, Heal, History -1, Insight, Intimidate +2, Nature, Religion -1, Athletics +1
FEATS
Level 1: Black Arrow Style
POWERS
Rogue at-will 1: Deft Strike
Rogue at-will 1: Sly Flourish
Rogue encounter 1: Shadow Strike
Rogue daily 1: Blinding Barrage
ITEMS
Sling, Sling Bullets (20), Leather Armor, Dagger
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Your combat style and tactics allows you to hide easily when there is cover or concealment, even when having to flee from melee.
I have chosen the sling, because i don´t know how blinding barrage should work with a crossbow.

And if your DM allows, you can use the sling to fire off some alchemical items.
You can as well use the standard rogue weapon talent and use shurikens (the most versatile build actually), but sling is more halflingy.
The most powerfull option is asuperior crossbow, but IIRC you can´t use it as halfling.
Your next feats would be backstabber and light shield proficiency for extra damage and protection. The rest is up to your taste.