Here's a simple idea. The percentage of damage you do it a person is the percentage of the total gold that you get. So if three archers all shoot the same guy and do the same damage, killing him, then all three archers get 1/3rd of the total gold.
If I swing my sword at a horseman and get him down to 50% health and a lancer finishes him, I get 1/2 of the gold and the lancer gets 1/2 of the gold.
As for getting credit for the kill, the person with the highest percentage of damage gets 2 points, and the rest get 1 if they contributed over 20% damage.