Obtained simultaneously with Sinister Siblings. This is the final boss battle in Act 2, which takes place in Infected Forest. In both cases, all you need to do is beat Sadista. I just did the battle myself, and here's some pieces of advice:
1) She throws a huge shuriken at you from long distance, and it can deal a lot of damage if you're not careful.
2) She uses her pistols to wildly fire all over the place
3) Every time you decrease her health by 25% or so, she will spawn a bunch of Netherim enemies. The lower her health goes, the more of them she spawns.
4) There will be what I can best describe as purple circles that launch explosives all over the area, so be very careful where you step.
5) Bring a lot of potions with you, as well as plenty of gold if you want to respawn right there. Believe me, you will need it.
I hope this helps you guys out