Forgot something.

If you want to encourage a better mix of parties don't weaken or buff either individually. Give incentives like an extra LB box or faster LB build for melee jobs. Reward diversity by making it easier for more diverse groups to achieve the goal. Penalize stacked groups for LB damage...ie. if you have more than two of any job then 3rd only contributes 1/2 damage to LB 4th 1/4 etc.