My personal opinion on the subject is that they should double the amount of points needed for any given "Kill" challenge, but credit a point for both a Kill and a Kill Most Damage Dealt. Yes, this would mean if you got a Solo Kill, it would be worth 2 points. This would eliminate the controversy without making it any easier for anyone to finish the challenge.
But then, I'm wondering if OP's comment about "bonus damage" from the "fresh arm" are legit. I was under the impression that only damage you dealt with your weapons on the target counted toward damage for purposes of a KMDD. Edit: Case in point, you don't get bonus damage credit for a crit on an ammo slot, so you shouldn't get credit for the arm for blowing off a side torso.
In any case, as it stands, if you're trying for a KMDD rather than just a kill, it's better to aim for parts of your target which are not already damaged, thereby earning more damage credit before you finish an opponent off. That goal is in direct opposition to a goal of winning.
Edited by Throet, 01 February 2016 - 10:24 AM.