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 ======
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
Stealth +23, Thievery +19, Streetwise +12, Dungeoneering +17, Athletics +11, Acrobatics +19, Nature +17, Perception +17
Arcana +7, Bluff +7, Diplomacy +7, Endurance +8, Heal +12, History +7, Insight +12, Intimidate +9, Religion +7
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
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
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)
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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.
The effect will not happen until the trigger does.
Announcing that an ally is an enemy will not cause the effect to trigger unless you do so as the former ally is now moving into an affected square. If the ally is now an enemy she would be such until declared otherwise. And as such would be affected by any and all effects that effect enemies.
4e doesn't really have rules stating when an ally becomes an enemy so the conditions at which time this changes is entirely up to you as the DM. However, if there is a dispute between you and your players over this, it might be a good idea to develop a rule within your social contract (written or unwritten) that defines how you guys treat PVP and ally/enemy relations.
Note: be mindful of the mechanical implications of this, this is not a situation covered by the rules and a such, any house rule you make may come at the expense of system balance. Attacking allies is something that is used to great effect in certain optimization circles, allowing powers that target enemies to target former allies may have unpredictable results.
Specifics matter. For this feat, the attack is granted as a free action. In this specific case, your homebrew monster attacks, does damage, and stuns. The free action does not state that it interrupts the attack, and therefore resolves after the attack. Because the character is stunned, the resolution fails due to not being able to perform the action.
Immediate action states:
Therefore, as the feat has to wait for its trigger to resolve, and it's not explicitly an immediate interrupt, and stunned explicitly prohibits taking actions, the character stands there and looks stupid.
Specifically, unlike "no actions", "free actions" are things in themselves. They belong to the class "Action." There are no specific rules allowing free actions during stunned, therefore they are forbidden by the wording of stunned:
This kind of "don't bother this turn" is why the stunned condition produces such copious amounts of no fun at all and should never ever be part of an at-will attack.