Not evel close to legit. You lowered the guy`s health points each time you did it, so the last one killed him completely, and you wouldn`t show us this vid if you had one of someone doing it twice.
This is NOT legit, because it is not a GLITCH. You obviously do not understand that Halo 3 has health and that after you take them down to no shields like that so many times they can die from even 1 melee without a shot.