Cant exactly remember as I havent played Halo 4 in a while, but what I did was to catch them off guard. Shoot them with the DMR alot until they block or shoot back, then run away so that they dont follow you. They will still be damaged, so go back and do it over again untill they die. Its a long and tedious process, but I did the spartan ops on legendary difficulty and that was all that worked for me.