It is an addition to idea of strong self-healing creature who cant be killed by alone player. It is hard to make balance between healing rate and atk/def statistics of usual players. But probably we could implement that on other way: by using polinomial sum of players attack points.
I mean that if many players will attack creature, overall attack strength could be calculated in linear, polinomial or (why not? :-) even exponential form, like A[n]**n+a[n-1]**(n-1)+...+A**1+A - there n is number of attackers, for polinomial case. The reason is that creature cant defend himself from 360 degrees angle treats, unless it have many manipulators, like slime or ent. So creature's def could be decreased in case of team players work.
The same idea could be used in backward direction: one player could be able to attack many creatures at once. In that case his atk and def should be decreased in the same form as in case of multiple players vs single monster.
there is one possible exception from this - it is deathmatch. Im not sure that it is good idea if 7-8 high players would be able to fast kill gashadokuro before next one will appear.