i just did this, idk if it always works, but i tried a theory out someone said on this post. Fabio Iz B3AST said that if you die when the boss is close to death you will get the boss kill. i was running around meleeing enemies shooting the warden and totally ignoring him with my BR bayonet and was meleeing like crazy and when warden was near death, i tested it. ran up to him, meleed and died, and it gave me a medal for it after he died while i was in respawn.
Click the plus for details and there you will see it. So everyone get out there and melee warden before he dies and steal that kill. Kill steal evolved. i was at 1 of 7 on level 4 for warden, now im 2 of 7 for warden kills level 4.