There is some disagreement on the Wizards' forum on this as well.
There is a ruling from customer service in 2008.
You must be 3 squares AWAY from the
square in which you started your turn.
If you end your turn only two squares
away from where you started you will
not gain concealment with Shadow Walk.
A ruling from 2009 contradicts that.
In order for the Shadow walk to
trigger the warlock needs to move at
least 3 squares away from the starting
square. He does not need to finish his
move 3 or more squares away from the
starting square
Incidentally the PHB3 glossary explicitly defines movement as
Whenever a creature, an object, or an
effect leaves a square to enter
another, it is moving, whether that
move is done willingly or is forced.
This means shifting, teleporting, and
being pushed are all moves, for
example.
So if you were to teleport 3 squares on your turn (through a non-immediate power though please!) you should trigger Shadow Walk.
Assuming you can't get a theme, here's my suggestion for a damage-focused crossbow sniper:
====== Created Using Wizards of the Coast D&D Character Builder ======
level 14
Drow, Thief, Dread Fang
Background: Occupation - Criminal (+2 to Stealth)
FINAL ABILITY SCORES
Str 9, Con 12, Dex 24, Int 11, Wis 20, Cha 11.
STARTING ABILITY SCORES
Str 8, Con 11, Dex 18, Int 10, Wis 14, Cha 10.
AC: 29 Fort: 21 Reflex: 30 Will: 25
HP: 89 Surges: 7 Surge Value: 22
TRAINED SKILLS
Stealth +23, Thievery +19, Streetwise +12, Dungeoneering +17, Athletics +11, Acrobatics +19, Nature +17, Perception +17
UNTRAINED SKILLS
Arcana +7, Bluff +7, Diplomacy +7, Endurance +8, Heal +12, History +7, Insight +12, Intimidate +9, Religion +7
FEATS
Level 1: Ruthless Hunter
Level 2: Primal Sharpshooter
Level 4: Two-Fisted Shooter
Level 6: Backstabber
Level 8: Crossbow Expertise
Level 10: Weapon Focus (Crossbow) (retrained to Lasting Frost at Level 11)
Level 11: Primal Eye
Level 12: Wintertouched
Level 14: Silvery Glow
POWERS
Lolthtouched: Cloud of Darkness
Primal Sharpshooter: Grappling Spirits
Thief utility 1: Tactical Trick
Thief utility 1: Escape Artist's Trick
Thief utility 2: Fleeting Ghost
Thief utility 4: Sneak's Trick
Thief utility 6: Chameleon
Thief utility 7: Ambush Trick
Thief utility 10: Counter-Step
ITEMS
Frost Hand Crossbow +3, Feytouched Drowmesh +3, Bracers of the Perfect Shot (paragon tier), Assassin's Cloak +3, Eagle Eye Goggles (heroic tier), Frost Hand Crossbow +2, Potion of Healing (heroic tier)
====== Copy to Clipboard and Press the Import Button on the Summary Tab ======
I calculate DPR with to-hit chances, so my numbers will be much lower than yours.
Ranged basic attack: +23 v. AC, 1d8+26 cold damage
Combat advantage from 2 sources: wintertouched/lasting frost with a 1-(28-23)/20=75% chance and hiding based on invisibility or sneak's trick (any source of concealment will do)
When you start your turn hidden (for extra juice there, take persistent tail as your level 10 utility), you deal 4d8 as your sneak attack damage.
Tohit: (1-(28-(23+2))/20) = 85% hit rate, with backstabber means you generally won't miss.
DPR: .8*(4.5+26+4.5*4+5)+.05*(8+26+8*4+5+3.5*3+.85*(4.5+26+4.5*4+5)=49.14 DPR, all encounter long every encounter. Backstab to be used to turn misses into hits and you're a walking damage machine. AP of course gets you a second verse same as the first, so that's 100 damage on the first round. at 136 average HP for level 14, the slowed enemy (grappling spirits) gets a round to croak for help before you finish them in the second round. No dailies in the class which makes for an incredibly long-enduring character.
Free reroll on stealth checks from your neckslot with a standard action invis from your armor gives you all the stealth you really need. Especially with sneak's trick allowing remarkable stealth in places that would not otherwise be considered venues for sneaking. Average non-trained perception is +10 at 14, so a +23 with a reroll and an II to become re-hidden should provide for sufficient stealth.
For a nova-based assassin... I'll look at the executioner with big, sad, eyes... and go to the Ranger. Their damage simply cannot be beat. I'd personally choose a wind-rider, but that's because I find pure-damage frostcheese excruciatingly boring. The darkstrider gets you your hidden damage boost. Spending feats on skills is usually contraindicated. See here for a goodenough archer that just spams the damage.
If you choose to not go with thief (cunning sneak is slightly less good than sneak's trick, depending) or ranger, then executioner isn't a bad choice. You're trading damage for flavour, but it's very nice flavour. If you MC warlock and take shadow dancer then it's very nice, very very very very mobile flavour. Warlocks also make good assassins MC into assassin :)
Cunning sneak is just not that worth it in actual play. While it helps in theoretical "solo stealth" missions, it's hard being a meat&potatoes assassin. Bravo's a trap. If you're going PH1 rogue/ranger, focus on minor action attacks, multi-attacks, and frostcheese. They'll get you the most damage with the least tradeoffs. Darkstrider becomes better there, because 3+wis damage on multi-attacks very very quickly becomes stupid.
Don't forget to calculate to-hit in your damage potential.
Good luck. Feedback will be incorporated to refine requirements and my suggestions.
Best Answer
Some optimization, since the other answers cover the negative case well...
For stealth, consider "The rules of hidden club" as a fantastic discussion on how stealth actually works in D&D. You also cannot make a stealth check to hide at the beginning of an encounter because you're not a gnome nor have you taken a move action.
The trick you're looking for is shadow walk + cunning sneak for perma-stealth. Or, to allow for your specific goal, shadow walk + hidden sniper give gives you CA any time you have concealment.
You can get cunning sneak through hybrid talent, and shadow walk through accursed shadows. You may also want to consider the level 10 utility persistent tail.
Edit: Working off the "how can I optimize this" part of your question, I recommend:
The first mistake is combining melee and range. While it works in theory, it's very hard to pull off in practice. The cunning sneak, shadow walk combo is solid, though a touch gimmicky.
Here, you flit around the edges of the battlefield and drop sly flourish while you build up sufficient shrouds on your primary target, then you drop your nova-encounter.
Ki focus powers all of your attacks so you save on implements. Pixie flight will come in real handy and be a fascinating way to play through the game. As a ranged scout/striker who is tiny, you'll have all kinds of out-of-combat fun as well as in-combat. At the same time, your fey beast will provide a meatshield in combat so that you're not placing an undue load on your defender or leader.
DPR of this build:
`Sly Flourish: +13 v. AC / 1d6+13+2d6 = 80% accuracy, .75*(d(3,6)+13+2)+.05*(3*6+13+2+d(2,6))=21.12 = 3.40 round striker. Not particularly good, but above the 4 threshold so it works, especially with the stealth assets you bring.
And then you can dump 2-4 shrouds whenever appropriate on another mob, and then open as appropriate.
With more thought, I realized we were overthinking this. In the build above, you were using this whole complicated edifice to power stealth for combat advantage.
With the thief guide:
We get what you were working towards. First, with tactical trick, you get combat advantage against... anyone adjacent to an ally. You also get your full move with limited OAs, so you'll probably be able to find cover. With crossbow expertise, you ignore cover and superior cover and with two-fisted shooter you reload as free.
Therefore, every turn you can attack with CA, not dependent on a stealth roll.
Your DPR, counting your owlbear (who counts as an ally, BTW, and who can move when you take a move action, meaning you can target anyone you like with tactical trick): `+14 v. AC, 85% accuracy (counting perma-ca), 1d8+11+2d8 damage. with free RBA on crit) = 24.80 DPR. 2.90 round striker which puts you into the big leagues. With 2 uses of backstab and one rebound you'll hit every single turn (almost), and your DPR creeps ever higher.
You also get the stealth advantages from your build from sneak's trick, allowing you to sneak through deserted corridors (counts as cover) with impunity even if one enemy sees you because of chameleon. The melee version of this is even better due to the charge chassis it is built upon, but it doesn't seem the build you're trying to go for (and is sooo bloody boring).