Alekhine's Gun

Note: Since it is uncertain if your ranking and stats are overwritten each time you complete a level, it is recommended that you save this until after you’ve unlocked “Legendary.”

For this achievement, you need to play through any level and get the lowest ranking possible, which is ‘Maniac’. This can be achieved by killing everyone in the level with a loud weapon (silenced weapons, knives, the garrote and chloroform are ineffective for this achievement). The level that is recommended is Level 2, ‘A Mole Among Us,’ because it is short, the objectives are simple, and since it takes place in a hotel, you can hide in rooms and let the enemies funnel to you.

Regardless of which level you select, make sure you complete all the objectives before you go on your rampage. That way you can concentrate on killing everyone, including civilians, without it interfering with your goals. If you do decide to go with Level 2, keep in mind that Paolo Minelli, one of your objectives (who you steal documents from), must remain alive (he kind of looks like The Godfather). If you kill him, you will fail the level.

Of course, you can try this on any level you wish. Save your progress often so if you die (and you will), you can just reload your save and try a different approach.