To get legendary weapons you need to kill him by pressing that button during the story. If you make a save just before heading in to kill Jack and the Warrior, then copy the save file, you can keep killing him and he drops a legendary every time. This is according to a mate, I haven't tried it. Although I believe him as he sent a bunch of screenshots of the legendary weapons on the floor before picking them up.
I guess it depends on whether you class this to be cheating or not. Either way you will end up with a CC, so this way is just faster I guess, although you do then have to kill Jack and the Warrior each time.