If you're pulling from source and building your own,
your answer is inside zone/attack.cpp NPC:
eath() function, you'll see experience giving is based on the function hate_list.GetDamageTopOnHateList()
At a glance, it appears who gets credit is based on whoever does "top" damage, this is determined by the minimum of >50% of damage. It determines it with raid and grouping in mind. If you want to change this behavior, you'll need to edit either the NPC:
eath logic when it sets the killer *mob from damagetop, or change the GetDamageTopOnHateList function to return whatever whimsical determination you want to customize this to.
It's not really a fix or exploit, since this has been the behavior since ages. I recall a similar behavior on live at one point or another, who knows what it does now or what it did back during DoN. So coding it would require someone to determine what's "right" vs what it is now. So there's the areas to edit above, and you can change it to whatever you like.
As an example though, if you want to code in cannon's "have to be last hit + >50 dmg" logic, you can add after the line
Code:
Mob* killer = GetHateDamageTop(this);
Something like this
Code:
Mob* killer = GetHateDamageTop(this);
if (killer && killer_mob && killer->GetID() != killer_mob->GetID() && killer_mob->IsNPC()) killer = killer_mob;
This way, if the last hitter was an npc, and the top damage mob is not them, then the killer is now whoever last hit (aka an npc), meaning you would lose your exp/loot rights in cannon's logic not being matched.