Sam I am, a very hard achievement which need to kill 100,001 monsters/ enemies & much time of playing, is not hard now, do you want the achievement faster without hacks? just follow my steps:

1. Open the game
2. Choose "The Grand Cathedral" level on Tourist difficulty
3. Take a serious bomb
4. After when you took the serious bomb go forward much so stop you when you see many monsters
5. Save the game on "F6"
6. Activate the serious bomb
7. After what the serious bomb exploded, press "F9" and load from the last checkpoint and you will do that again, again & again until you will get the achievement

If I helped you please leave a like and comment! :-)