The best farming spot to do this is in Memory Sequence 7 in New Game Plus mode (second playthrough). After reaching the first checkpoint, there will be four enemies coming out a door. Jump in front of the door before they spawn and press Trianlge to blend in with the background and become invisible. Let the enemies walk past you and restart the checkpoint. You must pick up the purple helix boosts before the first checkpoint. Helix Blend is unlocked in Sequence 8 on the first playthrough. This means it will not be available on the Normal difficulty during Memory Sequence 7. You must be playing New Game Plus mode for this farming method to work.
Easy "Like A Silencer" achievement
You can easily farm a "Silencer Gold" rating in Memory Sequence 8. Complete the first section of the level, while silently and non-lethally taking out the two enemies on the third floor. After the style rating appears, restart the checkpoint before going down to the second floor. You can repeat this as many times as desired.
Easy "Like An Assassin" achievement
You can easily farm an "Assassin Gold" rating in Memory Sequence 1. Multikill the first two enemies in the game, and progress far enough so the style rating appears, then restart the checkpoint. You will respawn just before the first two enemies and can repeat this as many times as desired.
Easy "Master Of Stealth" achievement
To get the "Master Of Stealth" achievement, you must complete Memory Sequence 11 without walking into an enemy's field of view. When an enemy sees you and their vision cone turns yellow or red, you must immediately restart the checkpoint. It is acceptable if you throw fire crackers and noise darts to distract enemies. However, do not throw daggers at them, as they will automatically detect you. Just follow the exact path taken in the video. If you make a mistake, you can restart the checkpoint as many times as desired. After each section with enemies, you will get a bonus award and your statistics will be displayed. It must always say "Times Seen: 0". If it is not at zero, it means you have been detected and will have to restart the sequence from the beginning if you have already passed the next checkpoint.