What they should do is something like this...
Take the number of people at the peerless when it spawns plus 50%. Divide the peerless hit points by that number. Then increase each players odds of an arti drop as their damage done approaches that number. More then one person can receive an arti.
Ex) If a peerless has 100,000 hit points and there are 10 people killing it, then the amount of damage each person has to do to get the highest chance at an artis is 100,000/15= 6,667points of damage.
Now, if the max chance at an arti you can get is a 5% drop rate, then scale it according to how much of that damage they did. If they did 6,667 points of damage or more, then they get a 5% chance at a drop, if they did 3,334 points of damage they get a 2.5% chance at a drop. If they poped out and hit it once at the end for 60 points of damage, then they get 0.0045% chance at an arti.
This way, the more people that are there, the better. No more whining and crying about people not helping out. Theoretically all 10 people could get an arti off the same peerless, though it would be extremely unlikely. Those that hide and dont pull their fair share are only hurting themselves, and will actually want to work hard and do as much damage as possible to be sure they get the best chance at an arti. This would encourage people to work together, and if someone was still trying to be a jerk and only do a tiny bit of damage at the end, they would only be hurting themselves, and still actually helping the others. It would also allow for a pretty wide variance in individual skill without allowing those that cant pull their fair share of work to mooch of those that do.