If you have Trap Bolts for your Crossbow, you can execute this technique to kill Big Daddies easily. Give yourself about 20 feet (in-game, of course) and shoot your Trap Bolts so they hang between you and the location of the Big Daddy you want to kill. Then shoot a round with another weapon at the Big Daddy to anger it so it comes towards you. It will stumble across the Trap Bolts you've shot and die, since it only takes two of them to take one down.