After Pavel is captured, you are not to be found by an enemy or kill anyone. If you are found, you may reload a checkpoint. This does not affect the achievement. Be sure to pay attention to your blue light on your watch. If this light is on, you will be seen by enemies.
After moving past the first two enemies who move from the bulkhead doors, this large area has the majority of the enemies. You can knock out any enemy that is in your way. As long as the bodies aren't found, you can knock out as many as you want. This level is short, and shouldn't prove hard.