You DO NOT need to play on hard to get this achievement.
Watch the youtube video for more info on getting this achievement in one playthrough, on easy difficulty!
This is a glitch you can preform to get this achievement in less then 5 minute. You have to play to the end on easy difficulty of course.
1. Load the save "fountaine"
2.use the flamethrower with electric gel. It will be easy and much faster to kill him!
3. Once you go to drain the last container of adam, hit
AS SOON AS YOU PRESS
, PRESS THE
4. Change the difficulty to Hard5. YOU MUST PRESS THE START BUTTON FOR IT TO SAVE THE CHANGE
6. The achievement will pop when one of the "little sisters" give you the key to the city.
CONGRATS! you now have 1100!