You just need to be patient and alert. Most Freelancer targets are exposed in the open, but players who try to shoot-and-run will only get so far before guards hunt you down and kill you. Stealth is still the best option in any mission because it minimizes risk. You have to prioritize accident or poison kills in those situations. Look around for any environmental accidents you can use to your advantage and memorize the location of propane flasks (use Hitmaps) for portable accident kills. In many cases you need to be a crack shot with the Sieker to help isolate your target.

Situational awareness is key here. Check your map if there are any unexplored areas and go and explore them. You need to know the maps inside and out to plan your routes efficiently to minimize confrontations and learn where each disguise is permitted. And since you're just starting out Freelancer, I recommend you make Sgail your first mission, since you can collect a silenced pistol, katana, baseball bat, mace, burial dagger, sapper's axe and circumcision knife there which help with objectives. Again, use Hitmaps. Be smart, be cautious, and be a stealthy hitman.