The last hit XP system is just not a good one. I think ideally your party gets experience equal to the percentage of damage done to the creature. So if you're party takes off 60% of the Orc's HP you split 60% of its EXP value.
Alternatively you could split it so that it's evenly distributed among all members on your side of the fight. However, that seems potentially exploitable (drag enemies to guards, make them do work for you).
