12 is Better Than 6

You need to purchase all five perks to earn this achievement. The perks can be bought during the following episodes:

Arsenal ($44): Act 2 Episode 1
Dodger ($40): Act 3 Episode 7
Rat King ($32): Act 3 Episode 11
Cuirass (34$): Act 4 Episode 6
12 is Better than 6 ($34): Act 4 Episode 10

If you're short on cash you can run a few repetitive tasks which can be obtained from the vendor with the dollar sign ($) over him.

Just go to Act I episode 6 and throw a dynamite into the room where 3 baddies are playing cards. There are some bottles on their table that will break when the dynamite explodes. Then get yourself killed and repeat until the achievment pops. Took me around 5 minutes to do so, as I was missing only 50 of them.

You may or may not earn this achievement through natural progression of the story; if not, load up Act 1 Episode 6.

You'll start near some dynamite. Pick this up and proceed through the entrance. There's a room adjacent to the entrance you just entered with a door to the south. Inside, there are three enemies playing cards. Enter this room and throw your dynamite. You may not kill all three of them, if not just let yourself get killed from the on-rushing enemies to the south and reload the episode.

It shouldn't take you longer than 30 minutes to farm all 100 kills using this method.