i just found this, i have to get my gf points up to give it a try, but others have said it works
I finally beat the Ruby Level Black Widow in the Vegas - Lower Strip with zero help - here's how:
You need: full stamina (mine is 525) and full load of consumables: shivs and stun guns
(Health pack isn't needed) - the others you can get from the NY Associate level - loot jobs: using the job that expends 3 energy points, it should take about 1500 energy expenditure to get a full load.
You also need about 40 Godfather/Reward ponits/
Attack her: let her kill your Mafia then before she kills you shiv her once only.
Next let her crush you.
When you get crushed, it sends you back to Vegas - press attack again straight away. MW asks you if you want to refill health for 2 reward points - say yes.
Do the job again - this time dont shiv her
Repeat 3 or 4 times - each time by sacrificing yr mafia and getting crushed you will see her level drop about 1500 energy levels.
Every 3 or 4 times, shiv her again - ONCE!!
This way you will get her down to 1500 or so energy (from 25000)
then stun her 5 or 6 times and kill her.
That's it - no help, no health packs used and the bitch is dead.....
BTW - depending on your stats you may need more attmpts or less - also you may need to refill stamina - buts thats an extra 5 reward points...
Pass this on to whover needs help.