Once you've obtained the Dart Gun, you can try this technique. All that's required is Electrical Trap Darts (4-6 per Big Daddy). Take your 4-6 Trap Darts and line them down a hall or near where you can see a Big Daddy, making sure the path he'll take towards you is amply covered. Once that's done, shoot at the Big Daddy to get his attention and keep him running towards you. He'll run into each dart and kill himself.