Hum…
There could be a percentage contribution checking, plus player dependant loot (only you can see, I guess it is in the game, right?).
Depending on how much the player contributes on damage to kill the monster, the player could
be able to get more and more loot.
But instead of only rewarding well the player that did most damage, could reward well the players that did the highest contribution to the battle, and reward less the player who did less contribution.
What would be the contribution? Causing damage on the enemies, casting usefull buffs on players that does not have it (or have it’s buffs wearing out) and healing wounded allies, etc. Everything that helps in the combat.
Let’s say: 32 peoples were attacking the boss, and 18 of them did at least 12% of contribution or higher,
they get an awesome loot.
The other 4 peoples that did at least 6% of contribution on the boss, will get an average loot.
The rest, who run away or just hit the boss to get the loot, will get a bad loot.
Also, to make it fair to everyone, support characters could also get in that contribution checker, since they are also dangerous to the monsters that are attacking the player who got helped, plus they will also receive a nice reward aswell for helping their party stay alive and fight.
Beside the idea I just said would be really usefull on field bosses.