If you only need this achievement, I recommend that you follow the first video guide below, which skips some steps and is, therefore, faster than following the "Safety First" mission story.
This will also get you the following achievements if you don't have them yet:
The Chameleon achievement in HITMAN 2 worth 13 pointsChange your disguise during The Final Test.
The Silent Assassin achievement in HITMAN 2 worth 15 pointsComplete The Final Test unspotted. Kill only Jasper Knight, ensuring his body is not found.If you are also missing the following achievement, you can follow the video below instead to get this one knocked out during the same run.