This game has serious free-rider problems and it's getting worse. A suggestion.

[Krotoan]

Congrats on doing what i said in the beggining of my post, pointing out problems without offering solutions.
But sure, we can be more specific.

My point being that we have MANY suggestions for solutions and none of them address all the problems that are inherent in a "performance rewarding" system.

I never mentioned healing in my post. healers would be awarded by damage, not heals. Easier to deal it and solves your problem.

It was an example of a possible "average performance" for a class.. it doesn't need to be that.

Easy. Only damage in bosses count.
I'll do you a solid and offer 2 extra solutions.

1- Count only the average damage done on the last boss.
2- take the damage done to the 3 bosses, do an average, compare with the base average set.

Then people will only want to have partners who can buff their damage, demand greater uptime on their own rotations at the sacrifice of the other players (you think black mages don't move NOW.. hooboy) and then will dump on healers and tanks for downtime.

I'm not here to nitpick apart absolutely every solution though, just saying the problem ISN'T the implementation, it's the nature of the people you're giving this reward to. You need to solve for any possible pressure that could be put on other party members and considering it's a 4 player dungeon with classes that have strengths and weaknesses and party contribution factors, that isn't simple.