i like this idea.
we need something for the high level players and something to promote some fun group hunting, so it would really require something strong enough so that one player is not able to kill it alone. How about maby give the creture some abilitys that forces you to have to use the group attack mechanics. Maby make the creture able to make you loose inits or something like that so you would have no choice but to use the group abilitys, maby add some new group abillity.
Award and motivation for the kill might be a high amount of LP devided by the group and naturally higher level bones, meat, and hides.