I don't know how fast or hard-hitting you would want to be, but arbalest/heavy crossbow, steel bolts, and a fast 1H with good thrust fits the assassin idea pretty well.
Heavy Crossbow / Steel Bolts / Broad Short Sword or Espada Eslavona (or Dagger/Long Dagger if you feel pro).
You only need 12 strength to use all of that. Wear very light armor and use your large pool of attribute/skill points to get high agility, athletics, and weapon master. Don't skip the 4 points on power strike though.
With this build, you can circle around the map, taking longer paths that usually don't get used. Get behind the enemy team and start taking down people from behind. Try not to get surrounded by a mob. remember, you can outrun almost anyone else, so if you are outnumbered back off and find somewhere safe to reload your crossbow. If you can get right behind an enemy don't waste your bolt, a few quick sword thrusts will kill them.
Don't bother with shurikens or smoke bombs. They will only alert enemies that you are there.
edit: this tactic will also net you a bunch of tasty AFK kills when you reach the enemy spawn before anyone else on your team.