don't go anywhere near him until johnson gets rid of his shield. then you can lob a few grenades at him if you like or just go for the one-hit sword kill from behind. Try to be quick doing it though because the other elites do a good job of distracting him.
Usually on normal and pummel him with the energy sword, but on legendary its somewhat difficult to achieve without bieng killed. So just grab duel needlers, wait for Johnson to take away his shield and let rip. Repeat it and you should win...thats how i did it