I recommend for this achievement the easy difficulty. There are 3 scientists and all 3 are really close to the research papers objectives (safes). Don't kill any scientist or the achievement is gone (but you can reload a save).
Go behind a scientist and the option to knock out appears. Pick up the body and carry the scientists to the new target on your map.
Hide the body in the hay and make sure the game registers this correctly otherwise reload a savegame. Repeat these steps with the other 2 scientists. After you hide all 3 scientists continue / complete the primary objectives and come back to the location and exit with all 3.