Once you get the Grenade Launcher and the Proximity Mines, this technique can be employed. Find a red fuel tank (all over the place), and cover it with about 6 Proximity Mines or so. Next, use Telekinesis to lift the massive explosive device and hurl it at a Big Daddy. This deals massive damage, leaving you with minimal effort to finish the job. Just be careful not to have the bomb blow up before you get there. (The launcher upgrade that deals no damage to you is a great thing to have just in case.)